Dapagliflozin stimulates glucagon secretion at high glucose: experiments and mathematical simulations of human A-cells

Abstract

Glucagon is one of the main regulators of blood glucose levels and dysfunctional stimulus secretion coupling in pancreatic A-cells is believed to be an important factor during development of diabetes. However, regulation of glucagon secretion is poorly understood. Recently it has been shown that Na(+)/glucose co-transporter (SGLT) inhibitors used for the treatment of diabetes increase glucagon levels in man. Here, we show experimentally that the SGLT2 inhibitor dapagliflozin increases glucagon secretion at high glucose levels both in human and mouse islets, but has little effect at low glucose concentrations. Because glucagon secretion is regulated by electrical activity we developed a mathematical model of A-cell electrical activity based on published data from human A-cells. With operating SGLT2, simulated glucose application leads to cell depolarization and inactivation of the voltage-gated ion channels carrying the action potential, and hence to reduce action potential height. According to our model, inhibition of SGLT2 reduces glucose-induced depolarization via electrical mechanisms. We suggest that blocking SGLTs partly relieves glucose suppression of glucagon secretion by allowing full-scale action potentials to develop. Based on our simulations we propose that SGLT2 is a glucose sensor and actively contributes to regulation of glucagon levels in humans which has clinical implications.

Figures

Figure 1. Effect of dapagliflozin (10 nM) on glucagon secretion from human and mouse islets at different glucose concentrations.

The figure shows estimates from linear mixed-effects statistical modeling with error bars indicating standard errors. For each islet preparation (i.e., each individual or animal) 6–8 groups of size-matched islets were tested for each indicated condition from 3 donors for human data, and from 29 (1 mM glucose), 11 (6 mM glucose) or 18 (11 mM glucose) animals for mouse data.

Figure 2. Model simulation of electrical activity.

The glucose concentration was raised from 1 mM to 6 mM and 11 mM, followed by simulated dapagliflozin application, as indicated. Simulations were performed by changing the glucose parameter in the SGLT2 submodel, GSGLT2, from 1 mM to 6 mM to 11 mM, and by lowering the KATP conductance from 0.15 to 0.115 nS to model the increase in glucose concentration to 6 or 11 mM, since inhibition of KATP channel activity is maximal already at 6 mM glucose. Dapagliflozin application was assumed to block all SGLT2 transporters, which was modeled by setting the parameter n = 0.

Figure 3. Ion currents during simulated action potentials during low glucose, high glucose, and high glucose in the presence of dapagliflozin.

The upper panels show zooms on action potentials under the various conditions as indicated. The middle and lower panels show the simulated ion currents during the action potentials, as indicated in the legend. Note the different scales on the y-axes of the middle and lower panels.

Figure 4. Schematic representation of the control of glucagon secretion by KATP-conductance and SGLT2.