Roles of GTP and phospholipase C in the potentiation of Ca2+-induced insulin secretion by glucose in rat pancreatic islets

Abstract

Glucose can augment insulin secretion independently of K+ channel closure, provided cytoplasmic free Ca2+ concentration is elevated. A role for phospholipase C (PLC) in this phenomenon has been both claimed and refuted. Recently, we have shown a role for GTP in the secretory effect of glucose as well as in glucose-induced PLC activation, using islets pre-treated with GTP synthesis inhibitors such as mycophenolic acid (MPA). Therefore, in the current studies, we examined first, whether glucose augments Ca(2+)-induced PLC activation and second, whether GTP is required for this effect, when K+(ATP) channels are kept open using diazoxide. Isolated rat islets pre-labeled with [3H]myo-inositol were studied with or without first priming with glucose. There was a 98% greater augmentation of insulin secretion by 16.7 mM glucose (in the presence of diazoxide and 40 mM K+) in primed islets; however, the ability of high glucose to augment PLC activity bore no relationship to the secretory response. MPA markedly inhibited PLC in both conditions; however, insulin secretion was only inhibited (by 46%) in primed islets. None of these differences were attributable to alterations in labeling of phosphoinositides or levels of GTP or ATP. These data indicate that an adequate level of GTP is critical for glucose's potentiation of Ca(2+)-induced insulin secretion in primed islets but that PLC activation can clearly be dissociated from insulin secretion and therefore cannot be the major cause of glucose's augmentation of Ca(2+)-induced insulin secretion.

Triggering and amplifying pathways of regulation of insulin secretion by glucose.

“…Several pieces of evidence from two laboratories (58,66,74) have established that the amplifying action of glucose does not involve the phospholipase C-protein kinase C pathway in the mouse. Discordant conclusions have been reached for the rat ␤-cell (65,74,75), but the most thorough study of the question also concluded that phospholipase C activation cannot be the major cause of glucose amplification of insulin secretion (75).…”

Immunohistochemical localization of eight phospholipase C isozymes in pancreatic islets of the mouse

“…Glucose is the major stimulant for insulin secretion; however, the exact mechanism by which glucose induces insulin secretion still remains to be defined (Vadakekalam et al, 1997). Glucose can augment insulin secretion independently of K + channel closure, provided that the cytoplasmic free Ca 2+ concentration is elevated (Gembal et al, 1992).…”

“…Glucose can augment insulin secretion independently of K + channel closure, provided that the cytoplasmic free Ca 2+ concentration is elevated (Gembal et al, 1992). A role for phospholipase C (PLC) in this phenomenon has been both claimed (Turk et al, 1993;Zawalich and Zawalich, 1997) and refuted (Gembal et al, 1993;Vadakekalam et al, 1997). Kelly et al (1994) suggested that the enhanced phosphoinositide (PI) response was partially Ca 2+ -dependent and might involve activation of distinct isozymes of PLC expressed in the islets.…”

“…However, current information about the cellular localization of PLC isozymes has so far been mainly limited to the brain and related organs (Ross et al, 1989;Peng et al, 1997). Therefore the pancreatic islet appears to be an interesting region in which to study the distribution of PLC isozymes mainly to provide clues to solving the controversy concerning the role of PLC isozymes in insulin secretion (Gembal et al, 1993;Vadakekalam et al, 1997). Moreover, several protein kinase C (PKC) isozymes, the downstream molecules for the PLC signaling pathway, were found from mouse pancreatic islets (Knutson and Hoenig, 1997).…”

Site-Specific Localization of Protein Kinase C Isoforms in Rat Pancreas

“…Receptors for a number of hormones and growth factors are situated in the plasma membrane of both endocrine and acinar cells. These receptors are known to be coupled with G protein, PLC and PKC [8][9][10]. Recently, we demonstrated the site-specific distribution of PLC in rat pancreas by immunohistochemistry and Western blot analysis [11].…”