Isolated guinea pig coronary smooth muscle cells. Acetylcholine induces hyperpolarization due to sarcoplasmic reticulum calcium release activating potassium channels.

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Abstract
Smooth muscle cells, dispersed from the circumflex coronary artery of the guinea pig, were studied with the whole-cell configuration of the patch-clamp. The resting potential of about -40 mV was superimposed by spikelike hyperpolarizations (SLHs) up to -20 mV amplitude. The SLHs resulted from spontaneous transient outward currents (spontaneous TOCs) measured under voltage-clamp (-40 or -50 mV). Acetylcholine (ACh; 10 microM) increased SLHs and TOCs in amplitude and frequency. Atropine blocked the ACh effects. ACh-induced SLHs or TOCs were suppressed by bath application of tetraethylammonium (1 or 10 mM) or by cell dialysis with cesium, suggesting that they result from induction of potassium currents. In cell-attached patches, induction of currents through 130-pS potassium channels was recorded when ACh was bath-applied. An ACh-induced increase in intracellular [Ca2+] is suggested as a second messenger since SHLs and TOCs were suppressed by cell dialysis of 10 mM EGTA. ACh induced SHLs and TOCs in the absence of extracellular calcium. Intracellular application of 5 mg/ml heparin blocked ACh-induced TOCs. When the intracellular calcium stores were depleted by pretreatment with caffeine, the ACh effects were suppressed. Similarly, ACh pretreatment reduced the caffeine-induced outward currents. The results suggested that ACh augments calcium release from the sarcoplasmic reticulum, and the released calcium activates maxi potassium channels. In the single cell, calcium-activated potassium channels generate TOCs and SLHs that sum up to a hyperpolarization of the multicellular tissue.