Effect of Ca2(+)-channel agonists and antagonists on skeletal muscle sugar transport

The purpose of this study was to determine whether changes in cellular Ca2+ caused by Ca2(+)-channel modifiers affect skeletal muscle sugar transport. Rat soleus muscles were isolated, and sugar transport was measured monitoring efflux of 3-O-methylglucose (3-MG). Muscles were "loaded" with 3-O-[methyl-14C]methyl-D-glucose in Krebs-Ringer-bicarbonate (KRB) media and then sequentially washed in radioisotope-free KRB. Cellular Ca2+ was modified by adding a Ca2+ agonist (BAY K 8644) and/or Ca2+ antagonists (nifedipine, nitrendipine, diltiazem) to the "load" and "wash" media. Alterations in cellular Ca2+ were determined from soleus muscle 45Ca2+ uptake measurements. Addition of 0.25 microM BAY K 8644 enhanced insulin-mediated 3-MG transport and 1 microM nifedipine prevented the agonist effect. A high concentration of BAY K 8644 (6 microM) led to increased basal but attenuated insulin-mediated 3-MG transport. Nifedipine (4 microM) and diltiazem (20 microM) blocked the BAY K 8644-induced changes in basal and insulin-mediated 3-MG transport. Nifedipine (4 microM) also attenuated the stimulation of cellular Ca2+ uptake by 6 microM BAY K 8644. Nitrendipine was unable to reverse the changes in basal or insulin-mediated 3-MG transport caused by BAY K 8644. These results indicate that a low concentration of BAY K 8644 enhanced insulin-stimulated skeletal muscle sugar transport, whereas a high concentration of the agonist led to a diminished ability of insulin to stimulate sugar transport. The effects of the Ca2+ agonist were presumably mediated through modulation of cellular Ca2+ as these effects were blunted by the Ca2+ antagonists.