CONTRASTING EFFECTS OF BETAXOLOL AND PROPRANOLOL ON CA2-ACTIVATED CONTRACTIONS IN SKINNED FIBERS FROM CANINE CORONARY-ARTERIES AND VENTRICULAR MUSCLES()

The effects of some beta-adrenoceptor antagonists (atenolol, betaxolol , bunitrolol, labetalol, pindolol, and propranolol) on Ca2+ (pCa 5.8)- activated tension development in chemically skinned fibers from canine coronary artery and right ventricular trabeculae were studied. In ski nned coronary arteries, Ca2+-activated tension development was decreas ed by betaxolol and propranolol at concentrations of more than 10(-5) and 10(-4) M, respectively. The pCa-tension relationships were shifted to the right and down by betaxolol. In contrast, in skinned cardiac m uscle Ca2+-activated tension development was increased by betaxolol an d propranolol at the same range of concentrations as in coronary arter ies, with no change in maximum tension. The pCa-tension relation was s hifted to the left by betaxolol. Other beta-adrenoceptor antagonists ( atenolol, bunitrolol, labetalol, and pindolol) had no effect on Ca2+-i nduced contraction in either muscle type. These results indicate that among beta-adrenoceptor antagonists, only betaxolol and propranolol ca n directly modulate the Ca2+ sensitivity of myofilaments and have oppo site effects on the contractile system in canine cardiac and vascular smooth muscle.