On the factors which determine the time-courses of junction potentials in the guinea-pig vas deferens

Abstract

The time-courses of junction potentials and junction currents at the sympathetic neuroeffector junction of guinea-pig vas deferens were investigated using simultaneous intracellular and focal extracellular recording. Excitatory junction potentials produced in the smooth muscle cells following electrical stimulation of the sympathetic nerves had durations of 600–1000 ms and were prolonged in time-course compared with the underlying excitatory junction currents which had durations of 40–150ms. The time-course of the excitatory junction potential could be predicted accurately from the time-course of the underlying excitatory junction current by assuming that the smooth muscle cells were isopotential during the excitatory junction potential. In contrast, the time-course of the spontaneous excitatory junction potential, produced by the action of a single quantum of transmitter, was identical to the time-course of the underlying spontaneous excitatory junction current, both events having durations of 40–150ms. Local application of 10 −4 M adenosine 5′-triphosphate in the vicinity of an intracellular microelectrode produced depolarizations (“adenosine 5′-triphosphate potentials”) of durations ranging between 400 and 2500 ms. When adenosine 5′-triphosphate was applied close to the rim of an extracellular electrode, negative-going signals (“adenosine 5′-triphosphate currents”) were produced which reflected the time-course of postjunctional membrane currents evoked by adenosine 5′-triphosphate. Adenosine 5′-triphosphate currents were abolished by α,β-methylene adenosine 5′-triphosphate (10 −6M) but were unaffected by tetrodotoxin (3 × 10 −7 M or 10 −6 M) and the α-adrenoreceptor blockers prazosin (10 −6 M) and phentolamine (10 −6 M). The time-courses of adenosine 5′-triphosphate currents were similar to the time-courses of excitatory junction currents recorded in the same preparation. Adenosine 5′-triphosphate currents were generally brief compared with simultaneously recorded adenosine 5′-triphosphate potentials which had durations of ≥ 1000 ms. The time-courses of relatively brief adenosine 5′-triphosphate potentials (duration ⪯800 ms) followed the time-courses of the underlying adenosine 5′-triphosphate currents. These results indicate that the time-course of decay of the excitatory junction potential in the guinea-pig vas deferens is determined mainly by the passive membrane properties of the smooth muscle cells, the duration of underlying transmitter action being brief. However, the factors determining the time-course of adenosine 5′-triphosphate potentials are less clear, and may include passive membrane properties and diffusion of locally applied adenosine 5′-triphosphate. The results support the hypothesis that adenosine 5′-triphosphate mediates the junction potentials and currents at this sympathetic neuroeffector junction.