Modes of secretagogue-induced [Ca2+]i responses in individual chromaffin cells of the perfusedrat adrenal medulla

Abstract

Chromaffin cells in the perfused rat adrenal medulla were loaded with indo-1 for confocal image analyses. Resting levels of [Ca2+]i in chromaffin cells were similar and were stable with time. This is in contrast to the situation in isolated rat chromaffin cells, in which spontaneous oscillations of [Ca2+]i are known to occur. When chromaffin cells were stimulated for 3–4 min by high K+ or nicotine, [Ca2+]i increased to a peak in 20–30s and then declined rather smoothly. In contrast, chromaffin cells stimulated by muscarine or low pH (6.5) commonly exhibited irregular oscillations in [Ca2+]i. This provides additional evidence supporting the previous claim that muscarine and low pH evoke catecholamine secretion using partly shared mechanisms. Although muscarine and low pH were speculated to produce weaker responses in noradrenaline-secreting cells due to their selective stimulation of adrenaline secretion, no clear indications for segregation of cell types from [Ca2+]i responses to these stimulants were found. The perfused adrenal medulla loaded with Indo-1 was also employed for simultaneously monitoring integrated changes in [Ca2+]i(Ca responses) by conventional microfluorometry and in catecholamine secretion from a whole medulla (secretory responses). When the profiles of secretory responses were approximated by the kth power of the profiles of Ca responses, the k-values were estimated to be 2.2 and 2.3 for high-K+- and nicotine-elicited responses, respectively, whereas a k-value of 1.4 was obtained for both muscarine- and low-pH-elicited responses. An analysis showed that the significant difference in the k-value with these two classes of stimulants is accounted for by the stimulant-dependentpatterns of [Ca2+]i responses found in confocal image analysis.