Restoration of the A cell response to glucose in isolated rat islets of Langerhans

Inhibition of protein kinase A-induced glucagon synthesis and secretion by glucose

The control of glucagon biosynthesis and secretion in the pancreatic islet was examined in response to protein kinase A stimulation at various glucose concentrations. Forskolin plus 3-isobutyl 1-methylxanthine (IBMX) stimulated both glucagon synthesis and secretion at a glucose concentration equivalent to hypoglycemia (0.5 g/L, P<.001), but not at higher glucose concentrations (1.0, 2.0, and 4.0 g/L, P>.05). Destruction of B cells with streptozotocin or inhibition of glycolysis with mannoheptulose did not reverse the inhibitory action of high glucose (4.0 g/L) on the response of glucagon to forskolin plus IBMX. In contrast, citrate but not EGTA treatment permitted forskolin plus IBMX to stimulate glucagon synthesis and secretion (P<.05 and P<.001, respectively) in the presence of high glucose. We conclude that citrate can block the inhibitory action of glucose on the response of A cells to the protein kinase A pathway, possibly through its effects on an intracellular metabolic pathway.

Suppression of Ca2+ oscillations in glucagon-producing alpha 2-cells by insulin/glucose and amino acids

The cytoplasmic Ca2+ concentration ([Ca2+]i) was continuously monitored in single glucagon-producing alpha 2-cells isolated from the mouse pancreas and later identified by immunostaining. Up to 60% of the alpha 2-cells exhibited spontaneous [Ca2+]i oscillations (frequency 0.1-0.3/min) in a medium containing 3 mM glucose. In originating from a basal level of 60-100 nM, reaching peak values of 300-400 nM and promptly disappearing after blocking voltage-dependent Ca2+ channels with methoxyverapamil, the oscillations resembled those in insulin-releasing beta-cells stimulated by glucose. The oscillatory activity was suppressed when combining elevation of glucose to 20 mM with the addition of 2-2000 ng/ml insulin. Whereas 10 mM of L-arginine or l-glycine transformed the oscillations into sustained elevation of [Ca2+]i, there was no response to 1 mM tolbutamide or 0.1-1 mM gamma-aminobutyric acid. The observations that alpha 2-cells differ from islet cells secreting insulin and somatostatin in responding to adrenaline with mobilisation of intracellular calcium can be used for their rapid identification. It is suggested that the oscillations reflect periodic entry of Ca2+ due to variations of the membrane potential.

Chromium and other insulin sensitizers may enhance glucagon secretion: implications for hypoglycemia and weight control

Increased pancreatic beta-cell secretory activity usually is associated with decreased alpha-cell activity; stimulated beta-cells release gamma-aminobutyric acid, which hyperpolarizes alpha-cells, inhibiting glucagon release. Thus, insulin secretion and glucagon secretion are usually inversely coupled. This suggests that chromium and other insulin-sensitizing modalities, by down-regulating beta-cell activity, may increase glucagon secretion. Such an effect might play a role in the documented therapeutic activity of supplemental chromium and biguanides in reactive hypoglycemia, and might also be of benefit to dieters.

Cyclic AMP raises cytoplasmic calcium in pancreatic alpha 2-cells by mobilizing calcium incorporated in response to glucose

The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in individual guinea-pig pancreatic alpha 2-cells exposed to modulators of glucagon release. Addition of the stimulatory amino acid arginine resulted in a sustained increase in [Ca2+]i, whereas the inhibitor glucose had the opposite effect. Epinephrine, the beta-adrenergic agonist isoproterenol, the adenylate cyclase activator forskolin and 8-bromo-cAMP transiently raised [Ca2+]i provided that the cells had been pretreated with glucose. However, simultaneous presence of glucose was not required and the effect occurred even in the absence of extracellular Ca2+. Carbachol, the alpha 2-adrenergic agonist clonidine and the sulfonylurea tolbutamide lacked effects on [Ca2+]i. In addition to providing support for the concept that glucagon release is positively modulated by [Ca2+]i, the results demonstrate that cAMP raises [Ca2+]i in the alpha 2-cells by mobilizing calcium incorporated in response to glucose.

Role of second messengers in the regulation of glucagon secretion from isolated rat islets of Langerhans

The roles of diacylglycerol (DAG), cAMP and Ca2+ in mediating the stimulatory action of arginine on pancreatic A cells have been investigated using phorbol esters, forskolin, a Ca2+ ionophore and trifluoroperazine (TFP). 0.5 microM 4 beta-phorbol 12-myristate 13-acetate (PMA) which stimulated glucagon secretion by approximately 3-fold in the absence of arginine, was unable to enhance arginine-stimulated glucagon secretion. Higher concentrations (1 and 10 microM) of PMA were able to enhance glucagon secretion in the presence of 1.25, 2.5, 5 but not 10 mM arginine. Insulin secretion was enhanced by PMA under all the conditions tested. Arginine (10 and 20 mM)-stimulated secretion of glucagon and insulin were synergistically augmented by 20 microM forskolin. While the effects of forskolin plus PMA on the A cells were additive, the effects of the two agents on the B cells were synergistic. The responses of the A and B cells to arginine required extracellular Ca2+. Secretion of the two hormones was dose-dependently stimulated by A23187. Arginine-stimulated glucagon secretion but not insulin secretion, was dose-dependently inhibited by TFP. These results suggest that proposed cellular second messengers interact differently in the A and B cells, and DAG and Ca2+ may play pivotal roles in mediating the actions of arginine on the A but not B cells; cAMP may play a modulatory role in the A cell response to arginine.