Autocrine/paracrine modulation of calcium channels in bovine chromaffin cells

Applying 10-s pulses of 10 mM Ba2+ to resting or K+-depolarized (70 mM) bovine adrenal chromaffin cells superfused with a nominal 0Ca2+ solution produced a large catecholamine secretory peak. In contrast, pulses of 10 mM Sr2+ or Ca2+ did not induce secretion from polarized resting cells, and induced smaller and narrower secretory peaks from depolarized cells; the areas of the secretory peaks from depolarized cells were 1.87, 3.06 and 27.4 nA s, respectively, for Ca2+, Sr2+ and Ba2+. Ca2+ channel currents in isolated cells or in cells surrounded by other unpatched cells (cell cluster) were studied with either the continuous-flow or the flow-stop method. When applied to an isolated cell, flow-stop reduced the amplitude of ICa by 19%, ISr by 31%, and IBa by 53%, compared with the current amplitude measured under continuous-flow conditions. This decrease in current amplitude was accompanied by a pronounced slowing down of current activation and could be largely relieved by applying strong depolarizing prepulses (facilitation). Under continuous-flow conditions, 10 microM exogenous ATP reduced (about 50%) ICa, ISr and IBa similarly. On the other hand, the use of Na+ as a charge carrier through Ca2+ channels, or intracellular dialysis with 1 mM BAPTA prevented the modulation of current by flow-stop. In cell clusters, activating secretion from unpatched cells, by either 10 mM Ba2+, 100 microM acetylcholine or 70 mM K+, caused a pronounced slowing down of current activation, as well as a decrease of its magnitude in the voltage-clamped cell immersed in the cluster. Such modulation of isolated cells was not observed. These data are compatible with the idea that the secretory activity of adrenal medullary chromaffin cells "in situ" controls the activity of their Ca2+ channels through autocrine/paracrine mechanisms.