K+ Channels Which Contribute to the Acetylcholine‐Induced Hyperpolarization in Smooth Muscle of the Guinea‐Pig Submucosal Arteriole

Membrane potentials were recorded from submucosal arterioles (diameter, 30-80 microns) of the guinea-pig small intestine, using conventional microelectrode techniques. In control solution the resting membrane potential was about -73 mV, and the addition of 0.5 mM Ba2+ depolarized the membrane to about -43 mV. 2. ACh (10 nM to 10 microM), or substance P (0.1 microM), caused a membrane hyperpolarization in preparations which had been depolarized by Ba2+ but not in control preparations. ACh produced a sustained hyperpolarization, whereas substance P produced a transient hyperpolarization, without being affected by either nitroarginine (0.1 mM) or indomethacin (10 microM). 3. In the presence of 50 microM BAPTA (acetoxymethyl ester form), the membrane potentials were not altered in the control solution or in the presence of Ba2+, but Ba2+ caused a smooth depolarization of the membrane. Following this procedure, both ACh and substance P caused membrane depolarization instead of hyperpolarization, suggesting that the ACh- and substance P-induced hyperpolarization in arteriolar smooth muscle are intracellular Ca2+ dependent. 4. In short segments (200-500 microns) of arteriole, the time constant of electrotonic potentials produced by passing current pulses through the recording electrode was about 75 ms. The addition of Ba2+ increased both the input resistance and the time constant. 5. The hyperpolarizations produced by ACh or substance P were associated with a reduction in the amplitude and the time constant of electrotonic potential. 6. The reversal potential for the ACh-induced hyperpolarization, estimated from the current-voltage relationship, was about -86 mV, a value close to the equilibrium potential for K+. 7. In the presence of 50 nM charybdotoxin the hyperpolarization produced by ACh became transient and was reduced in amplitude: the residual response was further reduced by apamin (0.1 microM). The response produced by substance P was also reduced by 50 nM charybdotoxin: again the residual response was sensitive to 0.1 microM apamin. The hyperpolarizations produced by either ACh or substance P were insensitive to glibenclamide (10 microM) and 4-aminopyridine (1 mM). 8. It is suggested that in submucosal arterioles of the guinea-pig ileum, ACh- or substance P-induced hyperpolarizations of smooth muscle result from activation of both charybdotoxin-sensitive and apamin-sensitive K+ channels, with the former being predominant. The results are discussed in relation to the possible involvement of one or more endothelium-dependent hyperpolarizing factors in ACh- and substance P-induced hyperpolarization.