Detection and characterization of evoked quantal depolarizations in smooth muscle

The relationship between spontaneously generated quantal depolarizations and the nerve-stimulation elicited evoked depolarizations during neurotransmission in smooth muscle organs has remained enigmatic. This issue was explored by studying the effects of a presumptive intercellular uncoupling agent, 1-heptanol, on the synaptic or "junction" potentials of smooth muscle, using intracellular recording. In the guinea-pig vas deferens, heptanol was found to suppress the nerve-stimulation evoked excitatory junction potential (eEJP) reversibly while the spontaneous EJPs (sEJPs) persisted. However, during suppression of the eEJP in certain cells (active cells), relatively brief stimulus-locked depolarizations still occurred intermittently, even though the prolonged depolarization of the eEJP was abolished. Analysis of these heptanol-resistant evoked depolarizations revealed a close similarity with the properties of sEJPs. Since the sEJP is thought to represent the quantal unit of neurotransmitter action in smooth muscle, the heptanol-resistant depolarizations have been termed quantal EJPs (qEJPs). The authors' results show for the first time that the unitary depolarization underlying the syncytial eEJP of smooth muscle is an sEJP-like event. They discuss the significance of this finding in terms of the electrical behaviour of syncytial smooth muscle, and speculate on the possible biophysical effect by which heptanol action may give rise to qEJPs.