Catecholamine modulation of magnesium plasma levels in the rat.

Epinephrine (0.3 microgram/min) or isoproterenol (0.2 microgram/min) were infused for 40 min in rats to determine the role of catecholamines in the short-term control of plasma magnesium levels. Phentolamine (0.2 and 1 mg/kg) or propranolol (5 mg/kg), injected i.p. 10 min following the beginning of the infusion with catecholamines, were used to block alpha- or beta-adrenoceptors. Epinephrine alone elevated the systolic while decreasing the diastolic blood pressure and initially increased heart rate by 8% to 455 +/- 25 beats/min. Plasma potassium declined by 24% but magnesium levels remained constant. Following 0.2 mg/kg of phentolamine, systolic and diastolic blood pressure decreased by 40% and 55% to 95 +/- 11 and 39 +/- 3 mmHg, respectively. Potassium levels were further reduced by 8%. Administration of this dose of phentolamine increased magnesium levels by 8%. Magnesium levels were elevated by 19% to 2.38 +/- 0.20 mEq/l after 1 mg/kg of phentolamine, possibly due to excessive hemodynamic derangements. Propranolol reversed the hypokalemic and hemodynamic effects of the epinephrine infusion without altering plasma magnesium. Similarly, propranolol abolished the tachycardia and hypokalemia elicited by isoproterenol without affecting magnesium levels. These results suggest that catecholamines play no major role in the short-term control of plasma magnesium in the rat.