title: Role of sodium or calcium in electrical depolarization of feline colonic smooth muscle.
abstract: The aim of this study was to compare the role of Na+ and Ca2+ on the membrane potential and contraction of feline colonic circular smooth muscle. The changes in membrane potential were correlated with a change in tension using the double-sucrose gap technique. The estimated resting potential was -62.8 +/- 2.6 mV. A depolarizing current, passed during quiescent periods of the membrane, stimulated a spike potential and caused a concomitant increase in smooth muscle tension. The maximum depolarization rate of the active spike potential was rapid (580 +/- 75 mV/s) and unaffected by the amplitude of the depolarizing current. Reduction of extracellular calcium (0.0 mM Ca2+ plus 5 mM EGTA) or blockade of the calcium channels with verapamil (10(-6)M) slowed the maximum rate of membrane depolarization to 128 +/- 20 mV/s (P less than 0.001). The steady-state amplitude of the electrotonic potential and time constant also decreased in low-calcium solutions. The amplitude of smooth muscle contraction and the time between stimulus and onset of contraction were dependent on the amplitude of the depolarizing current and the concentration of extracellular calcium. Removal of sodium had no effect on the electrotonic potentials, maximum dV/dt of the spike, or the contraction of the smooth muscle. These studies suggest calcium plays a major role in the generation of an active spike potential, colonic smooth muscle contraction requires extracellular calcium and is associated with an active regenerative spike potential, and sodium plays a minor role in the generation of an active spike potential and in the initiation of a contraction in feline colonic smooth muscle.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002415:Cats; D003106:Colon / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D004558:Electric Stimulation; D005769:Gastrointestinal Motility; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000502:physiology*; D009994:Osmolar Concentration; D012964:Sodium / Q000502:physiology*; D014700:Verapamil / Q000494:pharmacology
chemical_list: D004533:Egtazic Acid; D012964:Sodium; D014700:Verapamil; D002118:Calcium