Antihypertensive effect of betaxolol, a cardioselective β-adrenoceptor antagonist, in experimental hypertensive animals

Betaxolol, a beta1-adrenoceptor antagonist, has an affinity for L-type Ca2+ channels

The effect of betaxolol on the specific binding of [3H]diltiazem and [3H]nitrendipine to rat cortical membranes was examined. Betaxolol inhibited specific [3H]diltiazem and [3H]nitrendipine binding with IC50 values of 19.7 and 46.3 microM, respectively. The effect of betaxolol on L-type Ca2+ channels showed little stereospecificity, since similar inhibitions of radioligand binding were observed with both racemic betaxolol and L-betaxolol. The dissociation kinetics of [3H]diltiazem were unaffected by 30 microM betaxolol, whereas it increased the [3H]nitrendipine dissociation rate, thus suggesting that betaxolol directly interacts with the benzothiazepine binding site and allosterically modulates the dihydropyridine binding site. Carteolol, propranolol and timolol were also found to inhibit both specific [3H]diltiazem and [3H]nitrendipine binding to rat cortical membranes, but with less potency than betaxolol. The ability of betaxolol to interact with L-type Ca2+ channels may have a role in its therapeutic effects in the management of systemic hypertension and in reducing neuronal death as occurring in glaucoma.

Contrasting effects of betaxolol and propranolol on Ca(2+)-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 skinned coronary arteries, Ca(2+)-activated tension development was decreased by betaxolol and propranolol at concentration 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 muscle Ca(2+)-activated tension development was increased by betaxolol and propranolol at the same range of concentrations as in coronary arteries, with no change in maximum tension. The pCa-tension relation was shifted to the left by betaxolol. Other beta-adrenoceptor antagonists (atenolol, bunitrolol, labetalol, and pindolol) had no effect on Ca(2+)-induced contraction in either muscle type. These results indicate that among beta-adrenoceptor antagonists, only betaxolol and propranolol can directly modulate the Ca2+ sensitivity of myofilaments and have opposite effects on the contractile system in canine cardiac and vascular smooth muscle.

Betaxolol, a beta 1-adrenoceptor antagonist, inhibits Ca(2+)-induced contraction in skinned renal artery of the dog

The effects of betaxolol on Ca(2+)-induced contractions in saponin-treated skinned canine renal arteries were examined. Betaxolol decreased the pCa (-log[Ca2+] M) 5.8-induced contraction at the same concentrations at which the drug relaxed the high K(+)-induced contraction in intact smooth muscles. The pCa-tension relation was shifted to the right and downward by betaxolol. Betaxolol inhibited the increase in Ca(2+)-induced contraction by calmodulin, and this effect of betaxolol was observed even at the concentration that had no effect when given alone. These results suggest that betaxolol decreases the Ca2+ sensitivity of the contractile system in skinned canine renal arteries, presumably by inhibiting calmodulin-mediated contractions.