EFFECT OF BETA(1)-ADRENOCEPTOR BLOCKADE IN RATS AT 5 BAR AMBIENT-PRESSURE

Conscious rats exposed to 5 bar (500 kPa) ambient pressure show increa sed total myocardial blood flow (TMBF) and enhanced cardiac contractil ity in spite of unaltered mean arterial pressure (MAP), heart rate (HR ), and cardiac output (GO). Four groups of awake, adapted rats were gi ven injections of atenolol at 1 bar air or 5 bar normoxic N-2, or both . Atenolol injected at 1 bar caused a marked reduction of HR, MAP, pea k left ventricular pressure (LVP), and rate of LVP rise (+dP/dt) and f all (-dP/dt). In spite of beta(1)-adrenoceptor blockade, ambient press ure rise increased HR, LVP, +dP/dt, -dP/dt, TMBF, and calculated cardi ac O-2 consumption (P < 0.05). A second atenolol injection at 5 bar ca used a brief reduction in HR but did not affect cardiac contractility. Rats receiving the first atenolol injection at 5 bar demonstrated unc hanged TMBF. We conclude that beta(1)-adrenoceptor blockade does not a nnul the increase in cardiac contractility associated with hyperbaria.