Properties of a constitutively active Ca2+-permeable non-selective 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 (Er) 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 Er 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 (Po) 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 Po, 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.