Whilst the phenomenon of an electrical resting membrane potential (RMP) is a central tenet of biology, it is nearly always discussed as a phenomenon that facilitates the propagation of action potentials in excitable tissue, muscle, and nerve. However, as ion channel research shifts beyond these tissues, it became clear that the RMP is a feature of virtually all cells studied. The RMP is maintained by the cell’s compliment of ion channels. Transcriptome sequencing is increasingly revealing that equally rich compliments of ion channels exist in both excitable and non-excitable tissue. In this review, we discuss a range of critical roles that the RMP has in a variety of cell types beyond the action potential. Whereas most biologists would perceive that the RMP is primarily about excitability, the data show that in fact excitability is only a small part of it. Emerging evidence show that a dynamic membrane potential is critical for many other processes including cell cycle, cell-volume control, proliferation, muscle contraction (even in the absence of an action potential), and wound healing. Modulation of the RMP is therefore a potential target for many new drugs targeting a range of diseases and biological functions from cancer through to wound healing and is likely to be key to the development of successful stem cell therapies.

Insulin-induced hypoglycemia is a major barrier to the treatment of type-1 diabetes (T1D). Accordingly, it is important that we understand the mechanisms regulating the circulating levels of glucagon - the body's principal blood glucose-elevating hormone which is secreted from alpha-cells of the pancreatic islets. Varying glucose over the range of concentrations that occur physiologically between the fed and fuel-deprived states (from 8 to 4 mM) has no significant effect on glucagon secretion in the perfused mouse pancreas or in isolated mouse islets (in vitro) and yet associates with dramatic changes in plasma glucagon in vivo. The identity of the systemic factor(s) that stimulates glucagon secretion remains unknown. Here, we show that arginine-vasopressin (AVP), secreted from the posterior pituitary, stimulates glucagon secretion. Glucagon-secreting alpha-cells express high levels of the vasopressin 1b receptor gene (Avpr1b). Activation of AVP neurons in vivo increased circulating copeptin (the C-terminal segment of the AVP precursor peptide, a stable surrogate marker of AVP) and increased blood glucose; effects blocked by pharmacological antagonism of either the glucagon receptor or vasopressin 1b receptor. AVP also mediates the stimulatory effects of hypoglycemia produced by exogenous insulin and 2-deoxy-D-glucose on glucagon secretion. We show that the A1/C1 neurons of the medulla oblongata drive AVP neuron activation in response to insulin-induced hypoglycemia. Exogenous injection of AVP in vivo increased cytoplasmic Ca2+ in alpha-cells (implanted into the anterior chamber of the eye) and glucagon release. Hypoglycemia also increases circulating levels of AVP in humans and this hormone stimulates glucagon secretion from isolated human islets. In patients with T1D, hypoglycemia failed to increase both plasma copeptin and glucagon levels. These findings suggest that AVP is a physiological systemic regulator of glucagon secretion and that this mechanism becomes impaired in T1D.

Hypoglycaemia is a major barrier to the treatment of diabetes. Accordingly, it is important that we understand the mechanisms regulating the circulating levels of glucagon -the body's principle blood glucose-elevating hormone which is secreted from alpha-cells of the pancreatic islets. In isolated islets, varying glucose over the range of concentrations that occur physiologically between the fed and fuel-deprived states (from 8 to 4 mM) has no significant effect on glucagon secretion and yet associates with dramatic changes in plasma glucagon in vivo. The identity of the systemic factor that stimulates glucagon secretion in vivo remains unknown. Here, we show that arginine-vasopressin (AVP), secreted from the posterior pituitary, stimulates glucagon secretion. Glucagon-secreting alpha-cells express high levels of the vasopressin 1b receptor (V1bR). Activation of AVP neurons in vivo increased circulating AVP, stimulated glucagon release and evoked hyperglycaemia; effects blocked by pharmacological antagonism of either the glucagon receptor or vasopressin 1b receptor. AVP also mediates the stimulatory effects of dehydration and hypoglycaemia produced by exogenous insulin and 2-deoxy-D-glucose on glucagon secretion. We show that the A1/C1 neurons of the medulla oblongata, which are known to be activated by hypoglycaemia, drive AVP neuron activation in response to insulin-induced hypoglycaemia.Hypoglycaemia also increases circulating levels of copeptin (derived from the same pre-pro hormone as AVP) levels in humans and this hormone stimulates glucagon secretion from isolated human islets. In patients with type 1 diabetes, hypoglycaemia failed to increase both plasma copeptin and glucagon. These findings provide a new mechanism for the central regulation of glucagon secretion in both health and disease.