Sinoatrial automaticity and transmembrane potentials in hamsters on a magnesium-deficient diet

We studied the effects of moderate Mg deficiency on sinus node automaticity and atrial muscle transmembrane potential in hamsters. The Mg-deficient hamsters (Mg-D) were fed a diet containing less than 2.0 mmol/kg Mg (0Mg) for 7 days. They were then fed 0Mg enriched with 10 mmol/kg Mg acetate for 12 days. Control animals (Mg-N) were fed a Mg-sufficient diet consisting of 0Mg supplemented with 100 mmol/kg Mg acetate. Sinoatrial preparations from Mg-N and Mg-D animals were superfused in vitro with Tyrode solution at 36 degrees C. Transmembrane potentials were recorded using intracellular microelectrodes. Preparations from Mg-D hamsters beat at a more rapid spontaneous rate than did those from Mg-N animals. A decrease in [Mg]0 enhanced the automaticity of Mg-N preparations, but not of Mg-D preparations. The negative chronotropic action of carbamycholine (CBL) was greater in the Mg-D than in the Mg-N preparations. When driven at a constant rate, the atrial muscle fibers of Mg-D hamsters repolarized more rapidly than did those of the Mg-N group. The resulting difference in the action potential duration (APD) was independent of membrane resting potential (MRP), amplitude of the action potential (AAP), and velocity of depolarization during phase 0 (Vmax), which were the same in both groups of preparations. The APD was shortened to a similar extent, proportionally, by 100 nM CBL in both preparations, while MRP, AAP, and Vmax remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)