VASOACTIVE-INTESTINAL-PEPTIDE ELEVATES PINEALOCYTE INTRACELLULAR CALCIUM CONCENTRATIONS BY ENHANCING INFLUX - EVIDENCE FOR INVOLVEMENT OF ACYCLIC GMP-DEPENDENT MECHANISM

Vasoactive intestinal peptide (VIP) receptor density is high in the pi neal gland, which receives VIP innervation and responds to VIP with a relatively small increase in cAMP and cGMP levels. In the present stud y, we show that VIP (5-200 nm) treatment increased the intracellular c alcium concentration ([Ca2+](i)) in 64% of isolated individual pinealo cytes; in comparison, norepinephrine (NE) elevated [Ca2+], in 93% of t he cells and produced more robust responses. Analysis of the role of s econd messengers indicated that [Ca2+](i) was strongly elevated by cGM P analogs, but not by cAMP analogs. The nitric oxide-releasing agent S -nitro-N-acetylpenicillamine and 2,2-diethyl-1-nitroxyhydrazine also e levated [Ca2+](i). Investigation of the mechanisms revealed that respo nses to VIP or 8-bromo-cGMP involved Ca2+ influx, as did the plateau c omponent of the response to NE; the large rapid component of the respo nse to NE, however, appeared to reflect release from intracellular sto res. Pharmacological studies indicated that the VIP-induced Ca2+ influ x was mediated by a retinal rod-type cyclic nucleotide-gated cation ch annel, expression of which was confirmed by reverse transcription-poly merase chain reaction analysis. These observations indicate that funda mentally different mechanisms generate the responses to NE and VIP. Th e dominant effect of VIP causing transient elevation of [Ca2+](i) appe ars to be through cGMP gating a I-cis-diltiazem-sensitive rod-type cyc lic nucleotide-gated cation channel. In contrast, the dominant effect of NE on [Ca2+](i) is due to enhanced Ca2+ release from intracellular stores; the plateau component is due to influx through a I-cis-diltiaz em-insensitive channel.