Glucagon amplifies tetrodotoxin-resistant Na+ oscillations in glucose-stimulated pancreatic beta-cells

The cytoplasmic concentration of Na+ ([Na+]i) was measured in individual mouse beta-cells with dual-wavelength microfluorometry using the indicator SBFI. The addition of 10 nM glucagon to a medium containing 11 mM glucose and 1.3 mM Ca2+ resulted in a 24% increase of [Na+]i often associated with superimposed oscillations. When replacing Ca2+ with 5 mM Sr2+, the presence of glucagon resulted in an increase of the amplitude of the [Na+]i oscillations with decrease of their frequency. Similar effects as with glucagon were obtained with 1 mM 8-Br-cAMP or 5 microM of the Ca2+ channel agonist BAY K8644. The glucose-induced oscillations of [Na+]i were resistant to 3 microM tetrodotoxin and disappeared after addition of 100 nM clonidine, 10 microM methoxyverapamil or 400 microM diazoxide. Studies of cell aggregates revealed the existence of well synchronized [Na+]i oscillations similar to those in individual beta-cells. The results provide evidence for tetrodotoxin-resistant [Na+]i rhythmicity in glucose-stimulated pancreatic beta-cells subject to regulation with cAMP.