Pressor response induced by clenbuterol treatment in immobilized normotensive rats.

Short-term treatment with clenbuterol [0.6 mg/kg-1 subcutaneously (s.c.) daily] produces a pressor effect in stressed rats after a period of immobilization (40 min). The stress applied markedly increases the plasma concentrations of norepinephrine (NE) and epinephrine. After bilateral adrenal demedullation, the increased levels of catecholamines and the hypertensive response obtained after clenbuterol treatment in the stressed animals were reduced to the values of the control rats. Clenbuterol treatment produced desensitization of the beta 2-adrenoceptor-mediated effect and thus reduced the vasodilator response induced by isoproterenol and increased the vasoconstriction produced by epinephrine but not that caused by NE. This desensitization may be responsible for the hypertensive response after clenbuterol treatment in stressed animals which is attenuated after adrenal demedullation. In conclusion, our results provide evidence that clenbuterol treatment induces pressor effect in normotensive animals under stress.