Effects of magnesium on isolated rabbit atria.

The cardiac effects of magnesium were studied on 40 spontaneously beating rabbit atria at 37 degrees C. Magnesium administered in doses of 30, 60, 120 and 240 mmol elicited dose dependent negative inotropic and chronotropic responses. Larger doses above 240 mmol caused complete atrial arrest. Further it was observed that propranolol with magnesium became more effective in blocking the positive chronotropic and inotropic responses to isoprenaline. The finding is interesting and suggestive that magnesium could be employed either alone or in combination with propranolol in treatment of arrhythmias which are non-responsive to propranolol.

Reduced beta adrenergic responsiveness in isolated rabbit atria during hypothermia.

Beta adrenergic agonist isoprenaline (8x10(-6)M) produced a 500% increase in inotropic and 90% increase in the chronotropic responses of isolated rabbit atria at 37 degrees C. On cooling the atria to 23 degrees C, these responses were significantly reduced to 87% and 30% respectively. Similar results were obtained with adrenaline, but isoprenaline was more potent. The positive chronotropic and inotropic responses to isoprenaline were effectively blocked by propranolol and practolol at 37 degrees C whereas at 23 degrees C these beta blockers were unable to block even minor positive responses obtained by isoprenaline at this temperature. On the contrary at 23 degrees C, phenylephrine (alpha adrenergic agonist) produced marked positive chronotropic and inotropic effects indicating enhancement of alpha adrenoceptor activity at lower temperatures. This also suggests that reduced beta receptor activity at lower temperature is not due to a generalised depression of adrenoceptors as a result of hypothermia. Rewarming of atria to 37 degrees C restored the beta adrenoceptor responsiveness to previous level. It appears that ambient temperature is important im maintaining normal beta adrenergic activity of the atria.