Vasoactive intestinal peptide potentiates and directly stimulates catecholamine secretion from rat adrenal chromaffin cells

Abstract

The actions of vasoactive intestinal polypeptide (VIP) on catecholamine secretion and changes in [Ca 2+] i in single rat chromaffin cells were studied using amperometry and Indo-1. Application of VIP prior to acetylcholine (ACh) or co-application of VIP and ACh enhanced secretion by 94% and 153% respectively, compared to ACh alone. [Ca 2+] i was increased by 17% when VIP was preapplied and by 73% upon co-application. Exposure to VIP before stimulation with 60 mM K + enhanced secretion by 68%, but not [Ca 2+] i. VIP application prior to DMPP and nicotine had no effect on [Ca 2+] i, but increased [Ca 2+] i signals to muscarine by 18%. VIP co-application potentiated only [Ca 2+] i responses to muscarine, by 28%. The effect of VIP on muscarine-induced [Ca 2+] i signals was mimicked by 8-Br-cAMP, and both were blocked by H-89, a protein kinase A inhibitor. Long-lasting increases in secretion accompanied by a sustained rise in [Ca 2+] i to VIP alone were seen in 55% of cells. Removal of Ca 2+ or addition of La 3+ inhibited both responses, while L-, N- and P-type Ca 2+ channel blockers were ineffective. SK&F 96365 inhibited VIP-induced secretion completely and rises in [Ca 2+] i by 75%. Neither 8-Br-cAMP nor 8-Br-cGMP evoked responses similar to VIP alone. Thus in rat chromaffin cells, VIP acts both directly as a neurotransmitter in provoking sustained catecholamine secretion in a cAMP-independent manner, and also by enhancing ACh-induced secretion, via a cAMP-dependent action involving muscarinic receptors.