Harrihar A. Pershadsingh

Parallel stimulation in adipocytes of the plasma membrane Ca2+-transport/(Ca2+ + MG2+)-ATPase system and mitochondrial pyruvate dehydrogenase by a supernatant factor derived from isolated plasma membranesi

Summary A factor derived from the supernatant of adipocyte plasma membranes which mimics the effects of insulin on mitochondrial pyruvate dehydrogenase (Kiechle, F.L., Jarett, L., Kotagal, N., and Popp, D.A. (1981) J. Biol. Chem. 256, 2945–2951) was shown to activate a high affinity (Ca2+ + Mg2+)-ATPase and ATP-dependent Ca2+ transport of adipocyte plasma membranes. Fractionation of the supernatant by G25 Sephadex chromatography separated the material into four inactive and one active fraction. All of the fractions behaved identically in terms of activation of pyruvate dehydrogenase, (Ca2+ + Mg2+)- ATPase, and Ca2+ transport, suggesting that the active fraction responsible for stimulating mitochondrial pyruvate dehydrogenase was also responsible for stimulating the plasma membrane Ca2+-dependent processes.

A calmodulin dependent CA2+-activated K+ channel in the adipocyte plasma membrane

Increased membrane permeability (conductance) that is specific for K+ and directly activated by Ca2+ ions, has been identified in isolated adipocyte plasma membranes using the K+ analogue, 86Rb+. Activation of these K+ conductance pathways (channels) by free Ca2+ was concentration dependent with a half-maximal effect occurring at 32 +/- 4 nM free Ca2+ (n = 7). Addition of calmodulin further enhanced the Ca2+ activating effect on 86Rb+ uptake (K+ channel activity). Ca2+-dependent 86Rb+ uptake was inhibited by tetraethylammonium ion and low pH. It is concluded that the adipocyte plasma membrane possesses K+ channels that are activated by Ca2+ and amplified by calmodulin.

The Role of Calcium in the Transduction of Insulin Action

The year 1983 has particular significance for progenitors of the role of calcium in neurohumoral action and cell regulation, since it was precisely 100 years earlier that Ringer’s classic paper, describing the vital role of calcium in the generation of mechanical events in the heart, was published.(1) Since then, calcium has been shown to be an essential regulator of a diverse constellation of fundamental intracellular processes.(2–5) A fairly large body of evidence has accumulated over the past 20 years that implies an important role of calcium in the molecular mechanism of action of insulin. Suffice it to say that a reexamination of the role of calcium in the action of insulin is thereby both timely and fitting, especially in view of recent developments in this area. These are the issues that are dealt with in this brief review of some new and interesting findings that may once again reemphasize the importance of calcium and permit its reconsideration as an essential factor in the mechanism of insulin action.

STIMULATION OF GLUCOSE TRANSPORT BY INSULIN, VANADATE, CONCANAVALIN A, HYDROGEN PEROXIDE, AND PHORBOL ESTER OCCUR BY A CALCIUM-DEPENDENT MECHANISM

Publisher Summary This chapter elaborates evidence that increased intracellular calcium may be the common basis for the stimulatory effects of these agents on glucose transport. The membrane permeant ester, quin2-AM, is rapidly accumulated intracellularly and hydrolyzed by esterases to form the impermeant tetracarboxylate chelator form, quin2, which remains trapped intracellularly where it binds to and inactivates calcium. The inhibitory effect of quin2 loading on insulin-stimulated glucose transport could be partially reversed by preincubation with the calcium ionophore, A23187, only when 1.2 mM calcium was also present in the medium. Inhibition was complete for con A and H2O2 and almost complete for vanadate and TPA (approximately 90%) at the highest quin2-AM concentration tested. Saturated diacylglycerols and their porbol ester analogues (for example TPA) are potent stimulators of the calcium and phospholipid dependent protein kinase, protein kinase C. The stimulation of glucose transport by insulin and other diverse compounds appears to involve a common calcium-dependent pathway that may be related to activation of protein kinase C.