THE MECHANISM BY WHICH PROCAINE INHIBITS CATECHOLAMINE SECRETION FROM BOVINE CHROMAFFIN CELLS

1 We have investigated the action of procaine on stimulus-secretion coupling in bovine adrenal chromaffin cells. 2 Procaine inhibited the catecholamine secretion evoked by 500-mu-M carbachol (CCh) with an IC50 of 35-mu-M and the associated calcium influx (IC50 60-mu-M). It inhibited the catecholamine secretion evoked by depolarization with high potassium by less than 20% even at the highest concentrations tested (3.2 mm). 3 The secretion evoked by CCh was associated with an increase in sodium influx. This evoked influx was also inhibited by procaine (IC50 80-mu-M). 4 This selective action of procaine on the CCh-evoked catecholamine secretion was investigated further by patch-clamp techniques. 5 In agreement with the ion flux studies, procaine inhibited the inward current evoked by CCh. Procaine also altered the spectral characteristics of the noise associated with the agonist-induced current by adding an additional high frequency component. The amplitude of this component showed an e-fold increase for a 55 mV membrane hyperpolarization. 6 Data from cell-attached patches showed that increasing concentrations of procaine produced a progressive fall in the mean channel open time and an increase in mean blocked time. This combination led to a decrease in mean burst length. In addition, P(open) was reduced by 50-mu-M procaine. These changes in channel conducting time were sufficient to account for the reduction in inward current. A limited study of the action of procaine on nicotinic channels in outside-out patches gave similar results. 7 The data were considered in relation to various schemes of anaesthetic-channel interactions. The data did not fit the sequential blocking model or the extended channel block model but could be fitted to a modified sequential blocking model in which the rate constant for channel reopening after block was itself subject to modulation by the anaesthetic and the blocked channel could close without passing through the open state.