Calbindin-D28k decreases L-type calcium channel activity and modulates intracellular calcium homeostasis in response to K+ depolarization in a rat beta cell line RINr1046-38

Calbindin-D28k, acts as a modulator of depolarization induced calcium transients in the pancreatic beta cell. However, specific mechanisms have not been defined. Here we show for the first time that the calcium binding protein calbindin-D28k acts by affecting calcium influx through voltage-dependent calcium channels in RIN pancreatic beta cells. Whole-cell patch–clamp recordings revealed that Ca2+ current amplitudes of calbindin-D28k expressing RINr1046-38 beta cells were smaller than the Ca2+ current amplitudes in control cells in response to depolarizing pulses. The peak current was observed at +20 mV and the average amplitude was ∼50 pA in the calbindin expressing cells compared to ∼250 pA in control cells. In calbindin-D28k expressing cells, the channels had enhanced sensitivity to Ca2+ dependent inactivation and currents decayed much more rapidly than in control cells. The Ca2+ channels affected by calbindin were found to have biophysical properties consistent with dihydropyridine-sensitive L-type calcium channels. In response to depolarizing concentrations of K+, calbindin expression caused a five-fold decrease in the rate of rise of [Ca2+]i and decay was slower in the calbindin expressing cells. Application of verapamil resulted in a drop in the [Ca2+]i signal to pre-stimulation levels indicating that the Ca2+ channel responsible for the depolarization evoked Ca2+ entry, modulated by calbindin, is the L-type. Co-immunoprecipitation and GST pull-down assays indicate that calbindin-D28k can interact with the α1 subunit of Cav1.2. We thus conclude that calbindin-D28k can regulate calcium influx via L-type calcium channels. Our findings suggest a role for calbindin-D28k in the beta cell in modulating Ca2+ influx via L-type voltage-dependent calcium channels.