Modulation of glucose induced oscillatory [Ca2+]i signals by nitric oxide in ob/ob mouse pancreatic beta-cells.
Nitric oxide (NO) has been embroiled in the regulation of insulin secretion from the pancreatic beta cells and hence for the pathophysiology of diabetes mellitus. The present study was undertaken to assess the effects of hydroxylamine, a nitric oxide donor, on Ca(2+) handling in individual mouse pancreatic beta cells. Cytoplasmic Ca(2+) concentration ([Ca(2+)]i) was measured using dual wavelength microfluorometry and the indicator fura 2. In the presence of 3 mM glucose hydroxylamine raised [Ca(2+)]i in 90% of cells and the response was insensitive to methoxyverapamil and also to the intracellular Ca(2+) ATPase inhibitor, thapsigargin. At 11 mM glucose, the [Ca(2+)]i oscillations were abolished by hydroxylamine in a dose dependent manner. The addition of high concentrations of hydroxylamine (100 microM and 1 mM) resulted in a rapid disappearance of the oscillations with suppression of [Ca(2+)]i to near baseline level in a reversible manner. However, 90% of the beta-cells preserved the oscillatory [Ca(2+)]i activity in the presence of 10 microM hydroxylamine. At sustained elevated [Ca(2+)]i, obtained by depolarization with non metabolizable agonist, tolbutamide (1 mM), there was no effect of hydroxylamine; moreover, the inhibitory effects of hydroxylamine was counteracted by tolbutamide, suggesting that the effects of hydroxylamine is mediated by inhibition of metabolism leading to opening of K(+)(ATP) channel and decrease in Ca(2+) influx. When [Ca(2+)]i was maintained at sustained elevated state by at 11 mM glucose in the presence of glucagon, hydroxylamine at lower concentrations (<or=100 microM) induced generation of oscillatory [Ca(2+)]i signals. The generation of [Ca(2+)]i transients in the presence of glucagon was also abolished by hydroxylamine. The inhibitory effect of 1 mM hydroxylamine couldn't be transposed to oscillation by methylene blue, an inhibitor of guanylate cyclase and formation of c GMP. Thus the restraining effect of NO donor is probably not mediated by activation of c GMP signaling pathway rather reflects more direct effect of NO. It is concluded that, NO plays a dual role-at high concentrations it inhibits oscillatory [Ca(2+)]i signals in glucose-stimulated pancreatic beta-cells whereas it can regenerate the oscillatory activity at certain low concentrations when [Ca(2+)]i response is non oscillatory.
Ahmed
Meftun
M
Department of Physiology, Ibrahim Medical College, Shahbag, Dhaka, Bangladesh. meftun@hotmail.com
D000818:Animals; D055592:Biophysical Phenomena; D002118:Calcium; D020013:Calcium Signaling / Q000187:drug effects; D003600:Cytosol; D005947:Glucose; D019811:Hydroxylamine; D050417:Insulin-Secreting Cells / Q000187:drug effects*; D051379:Mice; D009569:Nitric Oxide / Q000096:biosynthesis*; D020030:Nitric Oxide Donors; D009994:Osmolar Concentration; D015398:Signal Transduction
D020030:Nitric Oxide Donors; D019811:Hydroxylamine; D009569:Nitric Oxide; D005947:Glucose; D002118:Calcium