Annual Review of Physiology - Volume 59, 1997

M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type, it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation. It can be modulated by a large array of receptor types, and the modulation can occur either by suppression or enhancement. Modulation of M-current has dramatic effects on neuronal excitability. This review discusses the numerous second messenger pathways that converge on regulation of this current: in particular, two forms of regulation of the M-current, receptor-mediated modulation and the control of macroscopic current amplitude by intracellular calcium. Both types of regulation are discussed with reference to the modulation of single-channel gating properties.

Endocardial and coronary vascular endothelial cells release substances that modify the contraction of cardiac myocytes. The major and possibly the sole up-regulating substance is endothelin. Several down-regulating substances are secreted, but none has yet been specifically identified. The relative amounts of up- and down-regulating substances are related to tissue oxygen tension. As pO2 rises, the concentration of up- and down-regulating substances, respectively, increases and decreases. Endothelin increases isometric force and decreases actomyosin ATPase activity thus increasing the economy of conversion of chemical to hydrodynamic energy. Beta-adrenergic agonists increase ATPase activity through an endothelial cell-dependent mechanism, leading to decreased economy. Therefore, two endothelial cell-dependent systems exist for regulating contractile efficiency: One involving endothelin appears to optimize the contraction for efficiency; the other, the beta-adrenergic-mediated system, optimizes for power.

Blood flow interactions with the vascular endothelium represent a specialized example of mechanical regulation of cell function that has important physiological and pathological cardiovascular consequences. The endothelial monolayer in vivo acts as a signal transduction interface for forces associated with flowing blood (hemodynamic forces) in the acute regulation of artery tone and chronic structural remodeling of arteries, including the pathology of atherosclerosis. Mechanisms related to spatial relationships at the cell surfaces and throughout the cell that influence flow-mediated endothelial mechanotransduction are discussed. In particular, flow-mediated ion channel activation and cytoskeletal dynamics are considered in relation to topographic analyses of the luminal and abluminal surfaces of living endothelial cells.

External load plays a critical role in determining muscle mass and its phenotype in cardiac myocytes. Cardiac myocytes have the ability to sense mechanical stretch and convert it into intracellular growth signals, which lead to hypertrophy. Mechanical stretch of cardiac myocytes in vitro causes activation of multiple second messenger systems that are very similar to growth factor-induced cell signaling systems. Stretch of neonatal rat cardiac myocytes stimulates a rapid secretion of angiotensin II which, together with other growth factors, mediates stretch-induced hypertrophic responses in vitro. In this review, various cell signaling mechanisms initiated by mechanical stress on cardiac myocytes are summarized with emphasis on potential mechanosensing mechanisms and the relationship between mechanical loading and the cardiac renin-angiotensin system.

Physical forces of gravity, hemodynamic stresses, and movement play a critical role in tissue development. Yet, little is known about how cells convert these mechanical signals into a chemical response. This review attempts to place the potential molecular mediators of mechanotransduction (e.g. stretch-sensitive ion channels, signaling mollecules, cytoskeleton, integrins) within the context of the structural complexity of living cells. The model presented relies on recent experimental findings, which suggests that cells use tensegrity architecture for their organization. Tensegrity predicts that cells are hard-wired to respond immediately to mechanical stresses transmitted over cell surface receptors that physically couple the cytoskeleton to extracellular matrix (e.g. integrins) or to other cells (cadherins, selectins, CAMs). Many signal transducing molecules that are activated by cell binding to growth factors and extracellular matrix associate with cytoskeletal scaffolds within focal adhesion complexes. Mechanical signals, therefore, may be integrated with other environmental signals and transduced into a biochemical response through force-dependent changes in scaffold geometry or molecular mechanics. Tensegrity also provides a mechanism to focus mechanical energy on molecular transducers and to orchestrate and tune the cellular response.

Osmoreceptors regulate sodium and water balance in a manner that maintains the osmotic pressure of the extracellular fluid (ECF) near an ideal set point. In rats, the concerted release of oxytocin and vasopressin, which is determined by the firing rate of magnocellular neurosecretory cells (MNCs), plays a key role in osmoregulation through the effects of natriuresis and diuresis. Changes in excitatory synaptic drive, derived from osmosensitive neurons in the organum vasculosum lamina terminalis (OVLT), combine with endogenously generated osmoreceptor potentials to modulate the firing rate of MNCs. The cellular basis for osmoreceptor potentials has been characterized using patch-clamp recordings and morphometric analysis in MNCs isolated from the supraoptic nucleus of the adult rat. In these cells, stretch-inactivated cationic channels transduce osmotically evoked changes in cell volume into functionally relevant changes in membrane potential. The experimental details of these mechanisms are reviewed in their physiological context.

Practical limitations of the patch-clamp technique when recording mechanogated membrane ion channels are considered. Mechanical overstimulation of the patch or the cell from excessive suction/pressure protocols induces morphological and functional changes. In particular, the plasma membrane becomes decoupled from the underlying cytoskeleton to form either membrane blebs (cell-attached) or ghosts (whole cell). As a consequence, a membrane ion channel may show either a decrease or an increase in its native mechanosensitivity or even acquire mechanosensitivity. The effect varies with ion channel and cell type and presumably arises because of a disruption of membrane-cytoskeleton interactions. We consider that such disruptions are a pathological consequence of excessive mechanical stress, either during or after seal formation, rather than an immutable consequence of patch-clamp recording. By careful attention to the suction/pressure protocols during sealing and throughout recording, such artifacts can be avoided.

Although mechanosensory responses are ubiquitous and diverse, the molecular bases of mechanosensation in most cases remain mysterious.MscL, a mechano-sensitive channel of large conductance of Escherichia coli and its bacterial homologues are the first and currently only channel molecules shown to directly sense mechanical stretch of the membrane. In response to the tension conveyed via the lipid bilayer, MscL increases its open probability by several orders of magnitude. In the present review we describe the identification, cloning, and first sets of biophysical and structural data on this simplest mechanosensory molecule. We discovered a 2.5-ns mechanosensitive conductance in giant E. coli spheroplasts. Using chromatographies to enrich the target and patch clamp to assay the channel activity in liposome-reconstituted fractions, we identified the MscL protein and cloned the mscL gene. MscL comprises 136 amino acid residues (15 kDa), with two highly hydrophobic regions, and resides in the inner membrane of the bacterium. PhoA-fusion experiments indicate that the protein spans the membrane twice with both termini in the cytoplasm. Spectroscopic techniques show that it is highly helical. Expression of MscL tandems and covalent cross-linking suggest that the active channel complex is a homo-hexamer. We have identified several residues, which when deleted or substituted, affect channel kinetics or mechanosensitivity. Although unique when discovered, highly conserved MscL homologues in both gram-negative and gram-positive bacteria have been found, suggesting their ubiquitous importance among bacteria.

Genetic and molecular studies of touch avoidance in the nematode Caenorhabditis elegans have resulted in a molecular model for a mechanotransducing complex. mec-4 and mec-10 encode proteins hypothesized to be subunits of a mechanically gated ion channel that are related to subunits of the vertebrate amiloride-sensitive epithelial Na⁺ channel. Products of mec-5, a novel collagen, and mec-9, a protein that includes multiple Kunitz-type protease inhibitor repeats and EGF repeats, may interact with the channel in the extracellular matrix. Inside the cell, specialized 15-protofilament microtubules composed of mec-12 α-tubulin and mec-7 β-tubulin may be linked to the mechanosensitive channel by stomatin-homologous MEC-2. MEC-4 and MEC-10 are members of a large family of C. elegans proteins, the degenerins. Two other degenerins, UNC-8 and DEL-1, are candidate components of a stretch-sensitive channel in motor neurons. Implications for advancing understanding of mechanotransduction in other systems are discussed.