Annual Review of Physiology - Volume 61, 1999

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductances expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Ion channels are the unitary elements that underlie electrical activity of gastrointestinal smooth muscle cells and of interstitial cells of Cajal. The result of ion channel activity in the gastrointestinal smooth muscle layers is a rhythmic change in membrane potential that in turn underlies events leading to organized motility patterns. Gastrointestinal smooth muscle cells and interstitial cells of Cajal express a wide variety of ion channels that are tightly regulated. This review summarizes 20 years of data obtained from patch-clamp studies on gastrointestinal smooth muscle cells and interstitial cells, with a focus on regulation.

The main contributors to increases in [Ca²⁺]i and tension are the entry of Ca²⁺ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca²⁺ release from store sites in the cell by the action of IP3 or by Ca²⁺-induced Ca²⁺-release (CICR). The entry of Ca²⁺ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca²⁺ waves then spread from these hot spots, which results in a rise in [Ca²⁺]i throughout the cell. Spontaneous transient releases of store Ca²⁺, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations—frequent discharge sites (FDSs)—and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intestine. The IPANs that have been physiologically characterized are in the intrinsic myenteric ganglia. They are numerous, about 650/mm length of small intestine in the guinea pig, and communicate with each other through slow excitatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the muscle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and with ascending and descending interneurons. The circular muscle contracts as an annulus, about 2–3 mm in minimum oral-to-anal extent in the guinea pig small intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how they are programmed to generate the motility patterns that are observed.

Angina pectoris often results from ischemic episodes that excite chemosensitive and mechanoreceptive receptors in the heart. Ischemic episodes release a collage of chemicals, including adenosine and bradykinin, that excites the receptors of the sympathetic and vagal afferent pathways. Sympathetic afferent fibers from the heart enter the upper thoracic spinal cord and synapse on cells of origin of ascending pathways. This review focuses on the spinothalamic tract, but other pathways are excited as well. Excitation of spinothalamic tract cells in the upper thoracic and lower cervical segments, except C7 and C8 segments, contributes to the anginal pain experienced in the chest and arm. Cardiac vagal afferent fibers synapse in the nucleus tractus solitarius of the medulla and then descend to excite upper cervical spinothalamic tract cells. This innervation contributes to the anginal pain experienced in the neck and jaw. The spinothalamic tract projects to the medial and lateral thalamus and, based on positron emission tomography studies, activates several cortical areas, including the anterior cingulate gyrus (BA 24 and 25), the lateral basal frontal cortex, and the mesiofrontal cortex.

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the β-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the β-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via α1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and β-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

This review surveys a wide range of cellular and molecular approaches to strengthening the injured or weakened heart, focusing on strategies to replace dysfunctional, necrotic, or apoptotic cardiomyocytes with new cells of mesodermal origin. A variety of cell types, including myogenic cell lines, adult skeletal myoblasts, immortalized atrial cells, embryonic and adult cardiomyocytes, embryonic stem cells, teratoma cells, genetically altered fibroblasts, smooth muscle cells, and bone marrow–derived cells have all been proposed as useful cells in cardiac repair and may have the capacity to perform cardiac work. We focus on the implantation of mesodermally derived cells, the best developed of the options. We review the developmental and cell biology that have stimulated these studies, examine the limitations of current knowledge, and identify challenges for the future, which we believe are considerable.

Molecular chaperones, including the heat-shock proteins (Hsps), are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Hsps have received the most attention in model organisms undergoing experimental stress in the laboratory, and the function of Hsps at the molecular and cellular level is becoming well understood in this context. A complementary focus is now emerging on the Hsps of both model and nonmodel organisms undergoing stress in nature, on the roles of Hsps in the stress physiology of whole multicellular eukaryotes and the tissues and organs they comprise, and on the ecological and evolutionary correlates of variation in Hsps and the genes that encode them. This focus discloses that (a) expression of Hsps can occur in nature, (b) all species have hsp genes but they vary in the patterns of their expression, (c) Hsp expression can be correlated with resistance to stress, and (d) species' thresholds for Hsp expression are correlated with levels of stress that they naturally undergo. These conclusions are now well established and may require little additional confirmation; many significant questions remain unanswered concerning both the mechanisms of Hsp-mediated stress tolerance at the organismal level and the evolutionary mechanisms that have diversified the hsp genes.

Intercellular channels present in gap junctions allow cells to share small molecules and thus coordinate a wide range of behaviors. Remarkably, although junctions provide similar functions in all multicellular organisms, vertebrates and invertebrates use unrelated gene families to encode these channels. The recent identification of the invertebrate innexin family opens up powerful genetic systems to studies of intercellular communication. At the same time, new information on the physiological roles of vertebrate connexins has emerged from genetic studies. Mutations in connexin genes underlie a variety of human diseases, including deafness, demyelinating neuropathies, and lens cataracts. In addition, gene targeting of connexins in mice has provided new insights into connexin function and the significance of connexin diversity.

Subcellularly localized Ca²⁺ signals in cardiac and skeletal muscle have recently been identified as elementary Ca²⁺ signaling events. The signals, termed Ca²⁺ sparks and Ca²⁺ quarks, represent openings of Ca²⁺ release channels located in the membrane of the sarcoplasmic reticulum (SR). In cardiac muscle, the revolutionary discovery of Ca²⁺ sparks has allowed the development of a fundamentally different concept for the amplification of Ca²⁺ signals by Ca²⁺-induced Ca²⁺ release. In such a system, a graded amplification of the triggering Ca²⁺ signal entering the myocyte via L-type Ca²⁺ channels is accomplished by a recruitment process whereby individual SR Ca²⁺ release units are locally controlled by L-type Ca²⁺ channels. In skeletal muscle, the initial SR Ca²⁺ release is governed by voltage-sensors but subsequently activates additional Ca²⁺ sparks by Ca²⁺-induced Ca²⁺ release from the SR. Results from studies on elementary Ca²⁺ release events will improve our knowledge of muscle Ca²⁺ signaling at all levels of complexity, from the molecule to normal cellular function, and from the regulation of cardiac and skeletal muscle force to the pathophysiology of excitation-contraction coupling.

ATP-sensitive K⁺ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. By cloning members of the novel inwardly rectifying K⁺ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the receptors for sulfonylureas (SUR1 and SUR2), researchers have clarified the molecular structure of KATP channels. KATP channels comprise two subunits: a Kir6.0 subfamily subunit, which is a member of the inwardly rectifying K⁺ channel family; and a SUR subunit, which is a member of the ATP-binding cassette (ABC) protein superfamily. KATP channels are the first example of a heteromultimeric complex assembled with a K⁺ channel and a receptor that are structurally unrelated to each other. Since 1995, molecular biological and molecular genetic studies of KATP channels have provided insights into the structure-function relationships, molecular regulation, and pathophysiological roles of KATP channels.

Two potent hypotensive peptides, adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP), are encoded by the adrenomedullin gene. AM stimulates nitric oxide production by endothelial cells, whereas PAMP acts presynaptically to inhibit adrenergic nerves that innervate blood vessels. Complementary, but mechanistically unique, actions also occur in the anterior pituitary gland where both peptides inhibit adrenocorticotropin release. In the adrenal gland both AM and PAMP inhibit potassium and angiotensin II-stimulated aldosterone secretion. Natriuretic and diuretic actions of AM reflect unique actions of the peptide on renal blood flow and tubular function. In the brain AM inhibits water intake and, in a physiologically relevant manner, salt appetite. Both AM and PAMP act in the brain to elevate sympathetic tone, effects that mirror the positive inotropic action of AM in the heart. Cardioprotective actions in the brain and heart may be important counter-regulatory actions that buffer the extreme hypotensive actions of the peptides when released in sepsis. Thus the biologic actions of the proadrenomedullin-derived peptides seem well coordinated to contribute to the physiologic regulation of volume and electrolyte homeostasis.

In this article, we review the basic pharmacological and biochemical features of endothelin and the pathophysiological roles of endothelin in cardiovascular diseases. Development of receptor antagonists has accelerated the pace of investigations into the pathophysiological roles of endogenous endothelin-1 in various diseases, e.g. chronic heart failure, renal diseases, hypertension, cerebral vasospasm, and pulmonary hypertension. In chronic heart failure, the expression of endothelin-1 and its receptors in cardiomyocytes is increased, and treatment with an endothelin receptor antagonist improves survival and cardiac function. Endothelin receptor antagonists also improve other cardiovascular diseases. These results suggest that the interference with endothelin pathway either by receptor blockade or by inhibition of endothelin converting enzyme may provide novel therapeutic drugs strategies for multiple disease states.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Although the maturation of sexual function and reproduction occurs after birth, essentially all of the critical developmental steps take place during embryogenesis. Temporally, these steps can be divided into two different phases: sex determination, the initial event that determines whether the gonads will develop as testes or ovaries; and sexual differentiation, the subsequent events that ultimately produce either the male or the female sexual phenotype. A basic tenet of sexual development in mammals is that genetic sex—determined by the presence or absence of the Y chromosome—directs the embryonic gonads to differentiate into either testes or ovaries. Thereafter, hormones produced by the testes direct the developmental program leading to male sexual differentiation. In the absence of testicular hormones, the pathway of sexual differentiation is female. This chapter reviews the anatomic and cellular changes that constitute sexual differentiation and discusses SRY and other genes, including SF-1, WT1, DAX-1, and SOX9, that play key developmental roles in this process. Dose-dependent interactions among these genes are critical for sex determination and differentiation.

Synchronous activity among ensembles of neurons is a robust phenomenon observed in many regions of the brain. With the increased use of multielectrode recording techniques, synchronous firing of ensembles of neurons has been found at all levels in the mammalian visual pathway, from the retina to the extrastriate cortex. Here we distinguish three categories of synchrony in the visual system, (a) synchrony from anatomical divergence, (b) stimulus-dependent synchrony, and (c) emergent synchrony (oscillations). Although all three categories have been well documented, their functional significance remains uncertain. We discuss several lines of evidence both for and against a role for synchrony in visual processing: the perceptual consequences of synchronous activity, its ability to carry information, and the transmission of synchronous neural events to subsequent stages of processing.

Echolocating bats use audition to guide much of their behavior. As in all vertebrates, their lower brainstem contains a number of parallel auditory pathways that provide excitatory or inhibitory outputs differing in their temporal discharge patterns and latencies. These pathways converge in the auditory midbrain, where many neurons are tuned to biologically important parameters of sound, including signal duration, frequency-modulated sweep direction, and the rate of periodic frequency or amplitude modulations. This tuning to biologically relevant temporal patterns of sound is created through the interplay of the time-delayed excitatory and inhibitory inputs to midbrain neurons. Because the tuning process requires integration over a relatively long time period, the rate at which midbrain auditory neurons respond corresponds to the cadence of sounds rather than their fine structure and may provide an output that is closely matched to the rate at which motor systems operate.

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.