Role of Na/Ca exchange and the plasma membrane Ca2+-ATPase in beta cell function and death.

Recent progresses concerning the Na/Ca exchanger (NCX) and the plasma membrane Ca2+-ATPase (PMCA) in the pancreatic beta cell are reviewed. The rat beta cell expresses two splice variants of NCX1 and six splice variants of the 4 PMCA isoforms. At the protein level, the most abundant forms are PMCA2 and PMCA3, providing the first evidence for the presence of these two isoforms in a non-neuronal tissue. Overexpression of NCX1 in an insulinoma cell line altered the initial rise in cytosolic-free Ca2+ concentration ([Ca2+]i) induced by membrane depolarization and the return of the [Ca2+]i to the baseline value on membrane repolarization, indicating that NCX contributes to both Ca2+ inflow and outflow in the beta cell. In contrast, overexpression of the PMCA markedly reduced the global rise in Ca2+ induced by membrane depolarization, indicating that the PMCA has a capacity higher than expected to extrude Ca2+. Glucose, the main physiological stimulus of insulin release from the beta cell, has opposite effect on NCX and PMCA transcription, expression and activity, inducing an increase in the case of NCX and a decrease in the case of the PMCA. This indicates that when exposed to glucose, the beta cell switches from a low-efficiency Ca2+ extruding mechanism, the PMCA, to a high-capacity system, the NCX, in order to better face the increase in Ca2+ inflow induced by the sugar. To our knowledge, this is the first demonstration of a reciprocal change in PMCA and NCX1 expression and activity in response to a given stimulus in any tissue.