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.