Abstract
Adrenal medullary chromaffin cells undergo exocytosis of catecholamines to provide physiological and behavioural adaptations during stress. Several different stimuli are known to evoke catecholamine secretion from the chromaffin cells, but in each case, exocytosis is driven by a rise in the intracellular Ca2+-concentration ([Ca2+]i). Much of the current understanding has been obtained from research conducted on isolated chromaffin cells. Under these conditions, the adrenal medulla’s anatomical organisation and the interconnections between individual cells are lost. As such, the current study examined the stimulus-evoked Ca2+-responses of chromaffin cells in rat adrenal slices loaded with a Ca2+-sensitive indicator. The specific stimuli were selected because they either mimic the endogenous neuronal stimulation (nicotine and pituitary adenylate cyclase-activating peptide (PACAP)) or are established physiological paracrine regulators of adrenal medullary activity (histamine).
Under basal conditions, most (72%) of the recorded chromaffin cells maintained low [Ca2+]i while the remaining cells displayed moderate to high-amplitude Ca2+-spikes. Nicotine evoked an elevation in [Ca2+]i which peaked within 1-2 min of drug application. In approximately one-third of the recorded cells, this peak response declined to basal levels within 15-20 min despite the continued presence of nicotine, while in the remaining cells, [Ca2+]i remained above at a constant, elevated level following a brief decline from the peak. Histamine evoked Ca2+-responses from most of the recorded chromaffin cells (approximately 90%), with [Ca2+]i rising to peak within 1-2 min of its application. A slow, relatively linear decline back to basal followed this peak response, and in about half of the histamine responsive cells, this decline was interrupted by relatively large-amplitude downward fluctuations in [Ca2+]i. While the histamine-induced Ca2+-responses between cells were considerably diverse, they were surprisingly constant for individual chromaffin cells, where a second consecutive histamine exposure produced a very similar Ca2+-profile to the first. Application of PACAP to the adrenal slices resulted in an elevated [Ca2+]i in approximately half of the recorded chromaffin cells. Surprisingly, in all cases, this PACAP response was markedly delayed, with [Ca2+]i elevation not observed until 3-13 min after the addition of the peptide. Following this delay, which varied between slices but was consistent between cells within a slice, the Ca2+-signal remained elevated for the duration of PACAP exposure.
These results support the view of the adrenal medulla as an integrator of multiple physiological stimuli in the maintenance of body homeostasis. They confirm previous data from isolated chromaffin cells but also provide new insights into the stimulus-dependent Ca2+-responses of individual chromaffin cells within the anatomical organisation provided by a tissue slice. Intriguingly, while all three stimuli increased the chromaffin cell [Ca2+]i, their distinctive profiles suggested that in addition to stimulating exocytosis, different stimuli may also induce different alterations in the chromaffin cell behaviour.