ACTIVATION BY MEMBRANE STRETCH AND DEPOLARIZATION OF AN EPITHELIAL MONOVALENT CATION CHANNEL FROM TELEOST INTESTINE

The intestine of euryhaline teleosts is an important osmoregulatory organ which actively absorbs Na+, Cl- and water from the lumen. This ion-transporting epithelium experiences a variety of physical stimuli resulting from variations in luminal osmolality and distension and from peristaltic contractions. Using patch-clamp techniques in the inside-out configuration, single stretch-activated channels (SA channels) were identified and characterized. These SA channels had a conductance of about 67 pS in symmetrical solutions containing 140 mmol l-1 NaCl and were permeable to both Na+ and K+ (P(Na)/P(K) almost-equal-to 0.83) but not to anions. In excised, inside-out membrane patches, channel activity could be enhanced in the absence of membrane tension by strong depolarization of the membrane potential (V(m)) to between 0 mV and +90 mV, with V(o) [V(m) at which the single-channel open probability (P(o))=0.51 at +25.7 mV. In the presence of membrane tension, the voltage-dependence of channel activity was shifted into the physiological range of V(m). Each kPa (10 cmH2O) of applied pressure (DELTA-P) generated the same effect on P(o) as a membrane depolarization of 49 mV. Membrane tension also increased the single-channel current and single-channel conductance in a dose-dependent manner. The kinetic data suggest that this channel has two open states and three closed states. Both stretch- and depolarization-induced increases in P(o) were attributed to prolongation of the lifetime of the longer open state. Possible physiological roles for this channel include the cellular uptake of Na+ from the lumen as part of the salt and water absorptive process or a yet undefined involvement in cell volume regulation.