Some electrical properties of the rabbit anococcygeus muscle and a comparison of the effects of inhibitory nerve stimulation in the rat and rabbit

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Abstract
Simultaneous recordings of mechanical activity and membrane potential of individual smooth muscle cells were made in the rabbit anococcygeus muscle and the effect of field stimulation on these examined. In the absence of tone, the mean resting membrane potential was -48 mV. In the stretched muscle spontaneous tone and rhythmic activity quite frequently appeared and this was associated with depolarization of the muscle cells. The response to field stimulation depended on the frequency of stimulation, the level of membrane potential and the presence of myogenic tone. The usual response to single pulses or low frequency stimulation was a hyperpolarization of up to 30 mV (mean 14 .+-. 6.8 mV) after a latency of 185 ms and accompanied by muscle relaxation. Higher frequencies (over 8 Hz) produced an initial depolarization and an associated contraction followed the end of stimulation). Phentolamine (5 .times. 10-6 M) and guanethidine (10-6 M) blocked the initial depolarization and contraction but had no effect on hyperpolarization, muscle relaxation or rebound depolarization and contraction. The effect of field stimulation in the presence of guanethidine (4 .times. 10-5 M) was re-examined in the rat anococcygeus. Single pulses were ineffective, repetitive stimulation produced muscle relaxation but no hyperpolarization comparable to the rabbit. Any oscillations in membrane potential were damped during field stimulation and sometimes a small hyperpolarization was produced with a maximum amplitude of 13 mV and a mean of 1.9 .+-. 1.2 mV. The transmembrane potential at the peak of hyperpolarization in the rabbit was rarely more than -70 mV. Passive displacement of the membrane potential by current pulses altered the amplitude of the hyperpolarization and suggested that there was a reversal potential at between -80 and -90 mV. No change in input resistance was measured during inhibitory nerve stimulation in either the rabbit or the rat but measurements based on electrotonic potentials indicated a reduction in membrane resistance, small in the rat but greater in the rabbit. In both species muscle relaxation is apparently associated with an increase in ionic permeability and a move, at least in the rabbit muscle, towards an equilibrium potential of -80 to -90 mV. In view of the much smaller effect in the rat, it is not clear whether this is the cause or at least the sole cause of the muscle relaxation.