Abstract
Electrical responses to brief electrical stimulation were investigated in the cerebral artery of a guinea-pig using a microelectrode. A single brief stimulus (0.05ms) induced a spike potential followed by a depolarizing slow-potential, and these events were associated with muscle contraction. An outward current injected into the smooth muscle cell induced spike potential but failed to induce depolarizing slow-potential. These activities persisted in the presence of TTX (10-6M), guanethidine (5×10-6M), or atropin (10-5M). TEA (5mM) enhanced the amplitude of the spike potential, but not that of the depolarizing slow-potential. When the external Na was reduced, the membrane transiently hyperpolarized. During this period, the depolarizing slow-potential could be evoked. In a Cl-deficient solution, the membrane depolarized and the amplitude of the depolarizing slow-potential decreased. From these observations it is believed that the contribution of K, Na, or Cl is minor. In a 20mM-Ca solution, a brief stimulation induced neither spike potential nor depolarizing slow-potential, but did induce a hyperpolarizing slow-potential. The hyperpolarizing slow-potential was also induced in a Na-deficient solution, but only after completion of Na re-distribution across the membrane. These observations suggest that a substance released by brief stimulation produces a prolonged change in ionic conductances of the smooth muscle membrane, allowing the muscle to contract for a certain period.
