Constitutively active mutants of the beta1-adrenergic receptor.

We provide the first evidence that point mutations can constitutively activate the beta(1)-adrenergic receptor (AR). Leucine 322 of the beta(1)-AR in the C-terminal portion of its third intracellular loop was replaced with seven amino acids (I, T, E, F, C, A and K) differing in their physico-chemical properties. The beta(1)-AR mutants expressed in HEK-293 cells displayed various levels of constitutive activity which could be partially inhibited by some beta-blockers. The results of this study might have interesting implications for future studies aiming at elucidating the activation process of the beta(1)-AR as well as the mechanism of action of beta-blockers.

NOVEL EVIDENCE THAT BETA-ADRENOCEPTORS OF THE MEDIAL SEPTAL AREA REGULATE BLOOD-PRESSURE AND ELECTROLYTE BALANCE

We investigated the participation of the beta-adrenoceptors of the septal area (SA) in sodium and potassium excretion and urine flow. The alterations in arterial pressure and some renal functions were also investigated. The injection of 2.10(-9) to 16.10(-9)M of isoproterenol, through a cannula permanently implanted into the SA produced a significant dose-dependent decrease in urinary Na+ and K+ excretion and urinary flow. Pretreatment with 16.10(-9) M butoxamine antagonized the effect of 4.10(-9) M isoproterenol but pretreatment with 16.10(-9) M practolol did not abolish the effect of isoproterenol. The beta 2-agonist terbutaline and salbutamol (4.10(-9) M when injected intraseptally also caused a decrease in urine flow and in renal Na+ and K+ excretion. After injection of isoproterenol or salbutamol (4.10(-9) M) into the SA, the arterial pressure, glomerular, filtration rate (GFR) and filtered Nd were reduced while Na+ fractional reabsorption was increased. The results indicate that the beta 2-adrenoceptors of the SA play a role in the decrease of Na+, K+ and urine flow and this effect may be due to a drop in GFR and filtered Na+ and to the rise in tubular Na+ reabsorption.

Study of the contraction induced by norepinephrine and clonidine in the isolated guinea-pig ileum

Norepinephrine (NE) and clonidine produce a phasic, dose-dependent contraction of the isolated guinea-pig terminal ileum. The effect of NE was blocked by prazosin which produced a parallel rightward shift of the concentration-effect curve to NE, with a significant depression of maximum effects. Yohimbine and indomethacin noncompetitively blocked, whereas practolol potentiated, the contractile effect of NE. The contractile effect of clonidine was not antagonized by indomethacin or atropine. These results suggest that the isolated guinea-pig terminal ileum has excitatory receptors sensitive to clonidine stimulation and excitatory alpha receptors sensitive to blockade by prazosin, and that the activation of the latter may be related to the activation of endogenous prostaglandin synthesis.