The Journal of Neurophysiology Vol. 86 No. 2 August 2001, pp. 760-770
Copyright ©2001 by the American Physiological Society

Impaired Inhibitory G-Protein Function Contributes to Increased Calcium Currents in Rats With Diabetic Neuropathy

 ¹Veterans Affairs Ann Arbor Healthcare System, Geriatric Research Education Clinical Center, Ann Arbor 48105;  ²Department of Internal Medicine, University of Michigan Healthcare System, Ann Arbor 48109; and  ³Department of Pathology, Wayne State University, Detroit, Michigan 48201

Hall, Karen E., Jackie Liu, Anders A. F. Sima, and John W. Wiley. Impaired Inhibitory G-Protein Function Contributes to Increased Calcium Currents in Rats With Diabetic Neuropathy. J. Neurophysiol. 86: 760-770, 2001. There is a growing body of evidence that sensory neuropathy in diabetes is associated with abnormal calcium signaling in dorsal root ganglion (DRG) neurons. Enhanced influx of calcium via multiple high-threshold calcium currents is present in sensory neurons of several models of diabetes mellitus, including the spontaneously diabetic BioBred/Worchester (BB/W) rat and the chemical streptozotocin (STZ)-induced rat. We believe that abnormal calcium signaling in diabetes has pathologic significance as elevation of calcium influx and cytosolic calcium release has been implicated in other neurodegenerative conditions characterized by neuronal dysfunction and death. Using electrophysiologic and pharmacologic techniques, the present study provides evidence that significant impairment of G-protein-coupled modulation of calcium channel function may underlie the enhanced calcium entry in diabetes. N- and P-type voltage-activated, high-threshold calcium channels in DRGs are coupled to µ opiate receptors via inhibitory G_o-type G proteins. The responsiveness of this receptor coupled model was tested in dorsal root ganglion (DRG) neurons from spontaneously-diabetic BB/W rats, and streptozotocin-induced (STZ) diabetic rats. Intracellular dialysis with GTPS decreased calcium current amplitude in diabetic BB/W DRG neurons compared with those of age-matched, nondiabetic controls, suggesting that inhibitory G-protein activity was diminished in diabetes, resulting in larger calcium currents. Facilitation of calcium current density (I_DCa) by large-amplitude depolarizing prepulses (proposed to transiently inactivate G proteins), was significantly less effective in neurons from BB/W and STZ-induced diabetic DRGs. Facilitation was enhanced by intracellular dialysis with GTPS, decreased by pertussis toxin, and abolished by GDPS within 5 min. Direct measurement of GTPase activity using opiate-mediated GTP[³⁵S] binding, confirmed that G-protein activity was significantly diminished in STZ-induced diabetic neurons compared with age-matched nondiabetic controls. Diabetes did not alter the level of expression of µ opiate receptors and G-protein  subunits. These studies indicate that impaired regulation of calcium channels by G proteins is an important mechanism contributing to enhanced calcium influx in diabetes.