Ca²⁺–Secretion Coupling Is Impaired in Diabetic Goto Kakizaki rats

The Goto Kakizaki (GK) rat is a widely used animal model to study defective glucose-stimulated insulin release in type-2 diabetes (T2D). As in T2D patients, the expression of several proteins involved in Ca²⁺-dependent exocytosis of insulin-containing large dense-core vesicles is dysregulated in this model. So far, a defect in late steps of insulin secretion could not be demonstrated. To resolve this apparent contradiction, we studied Ca²⁺–secretion coupling of healthy and GK rat β cells in acute pancreatic tissue slices by assessing exocytosis with high time-resolution membrane capacitance measurements. We found that β cells of GK rats respond to glucose stimulation with a normal increase in the cytosolic Ca²⁺ concentration. During trains of depolarizing pulses, the secretory activity from GK rat β cells was defective in spite of upregulated cell size and doubled voltage-activated Ca²⁺ currents. In GK rat β cells, evoked Ca²⁺ entry was significantly less efficient in triggering release than in nondiabetic controls. This impairment was neither due to a decrease of functional vesicle pool sizes nor due to different kinetics of pool refilling. Strong stimulation with two successive trains of depolarizing pulses led to a prominent activity-dependent facilitation of release in GK rat β cells, whereas secretion in controls was unaffected. Broad-spectrum inhibition of PKC sensitized Ca²⁺-dependent exocytosis, whereas it prevented the activity-dependent facilitation in GK rat β cells. We conclude that a decrease in the sensitivity of the GK rat β-cell to depolarization-evoked Ca²⁺ influx is involved in defective glucose-stimulated insulin secretion. Furthermore, we discuss a role for constitutively increased activity of one or more PKC isoenzymes in diabetic rat β cells.