Actions of nitroglycerine on smooth muscles of the guinea-pig and rat portal veins.

1 Effects of nitroglycerine (NG) on the electrical and mechanical activities of smooth muscle cells of the rat and guinea-pig portal veins were studied by a microelectrode technique and an isometric tension recording method.2 The membrane potentials of smooth muscle cells in the rat and guinea-pig were -44.4 mV and -47.6 mV, respectively. In both species the smooth muscle cells generated spikes as burst discharges.3 In the guinea-pig portal vein, NG (2.8 x 10(-8) M) produced biphasic potential changes, an initial transient hyperpolarization followed by depolarization. The hyperpolarization suppressed and depolarization enhanced spike activities.4 From the changes in the membrane potential produced by NG in various concentrations of [K](o), [Na](o) or [Ca](o), it is postulated that the hyperpolarization is due to an increase in the K-permeability and the depolarization is due to an increase in the Na-permeability of the membrane.5 NG (2.8 x 10(-5) M) had no effect on the membrane activity of the rat portal vein.6 NG consistently suppressed mechanical activities generated in both tissues. The minimum concentration required to suppress the mechanical activity was lower in the guinea-pig than in the rat portal vein.7 NG suppressed the contraction due to noradrenaline more than that evoked by excess [K](o) in both species.8 From these experiments, it is concluded that NG relaxes the muscular bed of portal veins of both species. In the rat portal vein, suppression of mechanical activity had no causal relation to the membrane activity. In contrast, in the guinea-pig portal vein, suppression of mechanical activity was slightly modified by changes in the membrane activity, i.e. hyperpolarization additively contributes to the relaxation and depolarization slightly suppresses the relaxation.