Membrane and contractile properties of the dog ciliary muscle

Membrane properties and excitation-contraction coupling mechanisms of the dog ciliary muscle were investigated by use of microelectrode and isometric tension recording methods. The mean resting membrane potential of the smooth muscle cell was -59.6 +/- 1.6 mV and the membrane was electrically quiescent. With applications of inward and outward current pulses, by the partition stimulating method (pulse duration, 1.0 s), electrotonic potentials, but not action potentials (spike) were evoked even in the presence of tetraethylammonium (TEA, 10 mM). The space and time constants calculated from the current-voltage relationship observed at various distances from the stimulating electrode were 0.43 +/- 0.07 mm (+/- s.d. n = 5) and 82.5 +/- 10.6 ms (+/- s.d. n = 4), respectively. Electrical field stimulation of short duration (50-300 microseconds) evoked excitatory junction potentials (e.j.ps) followed by twitch contraction, both of which were potentiated by neostigmine (10(-7) M) and abolished by tetrodotoxin (TTX, 10(-7) M) or atropine (10(-6) M). However, e.j.ps of amplitude over 3-5 mV did not evoke action potentials. Excess [K]o solution dose-dependently depolarized the membrane (the maximum slope of the depolarizations produced by a tenfold increase in [K]o plotted on a log scale was 52.5 mV) and evoked contractions. Atropine (10(-6) M) had no effect on the [K]o-induced membrane depolarization, however, it greatly reduced the amplitude of [K]o-induced contraction. Nisoldipine (5 X 10(-7) M) had no effect on the contraction evoked by field stimulation, however, this agent suppressed the [K]o-induced contraction to 78.3 +/- 6.8% of the control value. Combined application of nisoldipine and atropine completely inhibited the [K]o-induced contraction. Although the membrane depolarization evoked by carbachol (greater than 10(-7) M) was small (10 mV depolarization at 10(-5) M), the contraction evoked by 10(-5) M carbachol was larger than that evoked by 118 mM [K]o (185 +/- 50%, n = 9). Ca-free 3 mM EGTA-containing solution did not alter the resting tension, but greatly reduced the carbachol-induced contraction (to 8.6 +/- 2.3% of the control value). The present results indicate that the membrane of dog ciliary muscle behaves as an electrical syncytium, and that the contractile responses occur mainly through activation of the receptor-operated Ca channels and partly through activation of voltage-dependent Ca channels.