Akinori Yamaguchi

Calcitonin Gene—Related Peptide and Induction of Hyperglycemia in Conscious Rats In Vivo

The effect of calcitonin gene-related peptide (CGRP) on glucose metabolism was investigated in conscious and unrestrained rats in vivo. Intravenous injection of rat CGRP (5.67 and 0.567 nmol/kg) caused a significant, dose-dependent increase in plasma glucose concentration and a simultaneous dose-dependent increase in plasma insulin level. In contrast, plasma glucagon level was not changed. On the other hand, intravenous infusion of CGRP (46.6 pmol · kg−1 · min−1) decreased tolerance to intragastric administration of glucose (IGGTT). Plasma insulin response to IGGTT, however, was not affected by CGRP infusion. Moreover, although intravenous injection of CGRP (5.67 nmol/kg) elicited a significant increase in plasma epinephrine and norepinephrine concentrations, concomitant administration of epinephrine and norepinephrine, inducing a more prominent rise in plasma catecholamines than those induced by CGRP, affected neither plasma glucose nor insulin levels. Finally, plasma insulin levels obtained by simulating CGRP-induced changes in plasma glucose or glucose plus catecholamine levels by infusion of glucose or glucose plus catecholamines were not different from those induced by CGRP injection. These results suggest that CGRP has a hyperglycemic action that is not mediated by sympathetic outflow in conscious rats, and inhibition of insulin secretion, if any, does not play a major role in this hyperglycemic action of CGRP. We have demonstrated specific CGRP receptors linked to adenylate cyclase activation in rat liver plasma membranes; this hyperglycemic effect of CGRP in vivo may be partly due to its direct action on the liver.

Presence and release of calcitonin gene-related peptide in rat stomach

Immunoreactive (IR)-calcitonin gene-related peptide (CGRP) was identified throughout the entire stomach of rats, being most highly concentrated in the pyloric region, and the concentrations in muscular layers being higher than those in mucosal layers. In addition, IR-CGRP was also present in the venous effluent from isolated perfused rat stomach, and its release was stimulated by dibutyryl cyclic AMP or theophylline but not by glucagon. Gel chromatography as well as HPLC of both tissue extracts and gastric perfusate showed three identical major peaks of IR-CGRP, one of which coeluted with synthetic CGRP. These results suggest that CGRP in the stomach plays a role in the regulation of gastric function.

Prostaglandin E2 and F2α inhibit growth of human gastric carcinoma cell line KATO III with simultaneous stimulation of cyclic AMP production

The effects of prostaglandins (PGs) on the growth of human gastric carcinoma cell line KATO III were investigated. PGE2 as well as PGF2 alpha significantly and dose-dependently inhibited the growth of this gastric carcinoma cell line (PGE2 greater than PGF2 alpha). This inhibition of cell growth by the PGs was associated with the increase in cyclic AMP production (PGE2 greater than PGF2 alpha), whereas inositol-phospholipid turnover was not affected by either PGE2 or PGF2 alpha as assessed by the formation of 3H-inositol phosphates. Furthermore, the proliferation of these gastric carcinoma cells was also suppressed by the administration of forskolin as well as of dibutyryl cyclic AMP. These results suggest that PGE2 and PGF2 alpha inhibit the growth of cultured human gastric carcinoma cells KATO III via stimulation of cyclic AMP production.

Effects of islet amyloid polypeptide (amylin) and calcitonin gene-related peptide (CGRP) on glucose metabolism in the rat

In this study, we compared the effects of islet amyloid polypeptide (IAPP) and calcitonin gene-related peptide (CGRP) on glucose metabolism both in vivo and in vitro in the rat. Intravenous injection of rat CGRP caused a significant increase in plasma glucose concentration with a simultaneous increase in plasma insulin levels, whereas neither IAPP-NH2 nor IAPP-COOH had any effect. Moreover, intravenous infusion of CGRP decreased tolerance to intragastric administration of glucose (O-GTT) without altering plasma insulin levels, but again IAPPs had no effect. On the other hand, 125I-[Tyr0]rat CGRP specifically bound to the liver plasma membrane, and not only CGRP but also IAPP-NH2 dose-dependently displaced the specific binding of 125I-[Tyr0] CGRP, whereas IAPP-COOH had no effect. Conversely, CGRP as well as IAPP-NH2 but not IAPP-COOH evoked dose-dependent activation of adenylate cyclase in the membranes, and these effects were significantly inhibited by a CGRP receptor antagonist, human CGRP-I(8-37). However, neither CGRP nor IAPP-NH2 had any effect on glucose production in rat isolated hepatocytes. These results suggest that (1) IAPP-NH2 but not IAPP-COOH induces adenylate cyclase activation via CGRP receptors on rat liver plasma membranes, and (2) CGRP might not involve its action on the liver in the changes of glucose metabolism.

Glucose stimulates insulin release without altering cyclic AMP production or inositolphospholipid turnover in freshly obtained human insulinoma cells

Glucose, forskolin, IBMX and carbachol all stimulated insulin release from freshly obtained human insulinoma cells. In these same cells, cellular cyclic AMP levels were raised by forskolin and IBMX but not by glucose and carbachol. On the other hand, of all the insulin secretagogues examined, only carbachol stimulated the formation of 3H-inositol trisphosphate in these cells. Thus, in these insulinoma cells, glucose apparently induces insulin secretion without altering cyclic AMP production or inositolphospholipid turnover.