[Effect of islet activating protein on somatostatin-induced inhibition of cellular cyclic AMP content in isolated rat pancreatic acini].

Our previous study concerning guanine nucleotides regulation of labeled somatostatin binding has suggested that somatostatin receptors on pancreatic acinar cell membranes probably couple with the inhibitory guanine nucleotide regulatory protein (Ni). In order to clarify the possible role of Ni in mediating signal transduction of somatostatin in the pancreas, we further examined the effect of pretreatment with islet activating protein (IAP) on the inhibition of VIP-stimulated cellular cyclic AMP content by somatostatin in isolated rat pancreatic acini. Increasing concentrations of somatostatin decreased VIP-stimulated cellular content of cyclic AMP in the acini, with a maximal inhibition at 10(-8) M somatostatin. When pancreatic acini were pretreated with varying concentrations of IAP for 4 hours, the somatostatin-induced inhibition of cyclic AMP content was attenuated in a dose dependent manner by IAP pretreatment. Incubation of pancreatic acinar membrane with preactivated IAP and [32P] NAD resulted in labeling of a Mr = 41,000 protein band, consistent with alpha-subunit of Ni in many other cell types previously reported. On the other hand, a Mr = 41,000 protein band on SDS gel was reduced in a dose dependent fashion by IAP pretreatment, when acini were pretreated with increasing concentrations of IAP. These results suggest that only the Mr = 41,000 protein is a specific substrate in pancreatic acinar membranes for IAP-induced ADP-ribosylation. Furthermore, the reduction of 32P incorporation to Mr = 41,000 protein by IAP pretreatment occurred in parallel to decreases in somatostatin-induced inhibition of cellular cyclic AMP contents in pancreatic acini.(ABSTRACT TRUNCATED AT 250 WORDS)