Properties of a constitutively active Ca2+-permeable nonselective cation channel in rabbit ear artery myocytes

In smooth muscle, non-selective cation conductances contribute to agonist-evoked depolarisation and contraction, and in the present study using patch-pipette techniques we describe the properties of a constitutively active cation channel. With whole-cell recording in K+-free conditions, there was a spontaneous current with a reversal potential (E-r) that was altered by replacement of external Na+ by an impermeant cation, but not when external Cl- was replaced by an impermeant anion. The tonic cation inward current could be carried by Ca2+ ions and was greatly enhanced when the external Ca2+ concentration was reduced. In outside-out patches there was spontaneous cation channel activity that could be resolved into three conductance states of about 15, 25 and 40 pS, all with the same E-r as the whole-cell current. Kinetic analysis revealed that there were two open times of about 1 and 5 ms and that the currents displayed bursting kinetics with burst durations of approximately 5 ms and 25 ms. Removal of external Ca2+ ions increased the probability of channel opening (P-o) sixfold, which was associated with an increase in the longer burst duration. Bath application of the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol increased P-o, but phorbol 12,13-dibutyrate, which stimulates protein kinase C (PKC), reduced channel activity. In contrast, the PKC inhibitor chelerythrine increased the activity of channel currents. It is concluded that in rabbit ear artery myocytes there is a constitutively active Ca2+-permeable cation channel that is regulated by external Ca2+ ions and suppressed by tonic PKC activity. It is proposed that this mechanism may contribute to the resting membrane conductance and basal Ca2+ influx in this particular arterial preparation.