K+ channels which contribute to the acetylcholine-induced hyperpolarization in smooth muscle of the guinea-pig submucosal arteriole

1. Membrane potentials were recorded from submucosal arterioles (diameter, 30-80 mu m) 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 BA(2+) depolarized the membrane to about -43 mV. 2. ACh (10 nM to 10 mu M), or substance P (0.1 mu M), 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 mu M). 3. In the presence of 50 mu M 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 mu m) 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 mu M). The response produced by substance P was also reduced by 50 nM charybdotoxin: again the residual response was sensitive to 0.1 mu M apamin. The hyperpolarizations produced by either ACh or substance P were insensitive to glibenclamide (10 mu M) 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 charybdotoxoin-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.