Calcium Uptake and Protein Phosphorylation in Myenteric Neurons, Like the Release of Vasoactive Intestinal Polypeptide and Acetylcholine, Are Frequency Dependent

Abstract

The mechanism of the electrical-to-chemical decoding involved in the preferential release of the transmitters acetylcholine and vasoactive intestinal polypeptide (VIP) by electrical field stimulation at low (5 Hz) and high (50 Hz) frequencies was studied in superfused myenteric neurons. The stimulation-induced uptake of 45Ca2+ accompanying high frequency stimulation was markedly reduced by 10 microM nifedipine, a specific blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), as was also the preferential high-frequency release of VIP. By contrast, the 45Ca2+ uptake during low-frequency stimulation was somewhat lower per pulse, and neither this uptake nor the preferential release of acetylcholine occurring at this frequency was significantly reduced by nifedipine. These findings suggest that the release of acetylcholine and VIP involve different VSCCs. The pattern of in vitro protein thiophosphorylation in tissue extracts of differentially stimulated myenteric neurons involved polypeptides of 205, 173, 86, 73, 57, 54, 46, 32, 28, and 24 kDa and was also markedly stimulus and nifedipine dependent. This suggests that different phosphoproteins are involved during the frequency-dependent activation of the different Ca2+ channels and exocytotic mechanisms.