Low Physiological Level of Stimulation Elicits Exocytosis, not by Ca 2+ Influx, but by Suppression of Ca 2+ Syntillas in Mouse Adrenal Chromaffin Cells

Abstract

Catecholamine and neuropeptide exocytosis from adrenal chromaffin cells (ACCs) is controlled by sympathetic autonomic discharge. It has been thought that sympathetic discharge accomplishes this task exclusively by regulating Ca2+ influx through voltage-gated Ca2+ channels (VGCCs), which serves as a direct trigger for exocytosis. But our studies on spontaneous exocytosis in ACCs revealed the presence of Ca2+ syntillas, focal cytosolic transients mediated by ryanodine receptors (RYRs), and these had the surprising effect of inhibiting spontaneous exocytosis (Lefkowitz et al., 2009).Here we examine the role of syntillas under physiologic stimulation in ACCs using simulated action potentials (sAPs) designed to mimic native input at a frequency associated with basal sympathetic tone: 0.5 Hz. Stimulation at this frequency induces a general increase in the frequency and size of amperometric events comparable to that observed when syntillas were suppressed under spontaneous release conditions. Unexpectedly, we found that sAPs delivered at 0.5 Hz completely abolished Ca2+ syntillas within two minutes. Ca2+ arising from VGCCs was not enough to elevate the global [Ca2+]i as measured with fura-2. Hence, it appears that inhibition of syntillas by action potentials in ACCs is responsible for the increase in exocytosis at this level of stimulation.Funded by NIH grant HL21697 to JVW and AHA grant 0835580D to VD.Lefkowitz, J.J., K.E. Fogarty, L.M. Lifshitz, K.D. Bellve, R.A. Tuft, R. ZhuGe, J.V. Walsh, Jr., and V. De Crescenzo. 2009. Suppression of Ca2+ syntillas increases spontaneous exocytosis in mouse adrenal chromaffin cells. J Gen Physiol 134(4):267-280.