EFFECTS OF MANIPULATIONS OF CYTOPLASMIC PH ON THE MECHANICAL RESPONSES OF ISOLATED PORCINE CORONARY-ARTERIES

Isolated porcine coronary arteries contracted transiently when exposed to 20 mM Na salts of organic acids (acetate, propionate, butyrate and lactate) at constant external pH (pH0 = 7.4). The peak of these phasic contractions, completely inhibited by 1 mM amiloride, amounted to about 10% of that elicited by depolarization with 50 mM K0. The slope of the relaxing branch of the phasic contractions induced by 20 mM acetate in HEPES buffered physiological salt solution decreased from 1. 05 +/- 0.12 to 0. 49 +/- 0.07 mN/min (S. E. M.; n = 14), when NaO was reduced from 137 to 20 mM. In Na free HEPES-buffered PSS only sustained contractions were obtained. The EC50 values for the concentration-response curves for contractions induced by K-propionate (2-20 mM) in Tris-PSS were 8.2 +/- 0.5 mM, n = 6. After total isoosmotic replacement of external Na+ by sucrose in bicarbonate-buffered PSS, contractions induced by 20 mM K-propionate had the same height as those induced with 50 mM K+0. Longlasting exposure to 0 Ca0 PSS did not affect significantly contractions induced by 20 mM propionate. During an exposure to procaine (10(-4) M) acetate-induced contractions were transiently and significantly enhanced, when induced in bicarbonate buffered PSS, but not in HEPES buffered PSS. In normally polarized PCA preparations 20 mM NH+4 induced both sustained relaxations and polyphasic responses; in depolarized PCA preparations only polyphasic responses were evoked. The time course of the polyphasic responses to application and removal of weak bases (NH+4-pulse technique) or to removal and reintroduction of CO2/HCO-3 paralleled exactly the expected pH(i) perturbations, as they have been described in a great variety of cells and tissues, alkalinization leading however to relaxation, acidification to contraction. In presence of NH+4, a transient relaxation (phase 1) was immediately followed by an overshooting contraction (phase 2); at removal of NH+4 a rebound contraction (phase 3) was observed, reaching a peak and changing thereafter to a relaxation (phase 4), the slope of which was sensitive to variation of the external Na0 concentration. These mechanical effects were abolished by 1 hr exposure to 0 Ca PSS. Phase 2 was enhanced 2-5 fold in presence of TEA, Ba ions, Na3VO4 or beta-methyldigoxin. Contractile responses of PCA preparations activated by various agonists (acetylcholine, histamine and serotonin) to pH(i) manipulations were similar and generally amplified when compared to unactivated preparations. Both the recovery from the initial relaxation (phase 2) and the recovery from the rebound contraction (phase 4) of the biphasic responses to NH+4 application, which may reflect pH(i) regulation, are discussed in terms of a dual regulatory system involving the activation of an amiloride-sensitive Na-H-exchanger for recovery from acid loads and of an SITS-sensitive Cl-HCO3-exchanger for recovery from alkaline loads. The influence of pH(i) changes on the contractility of porcine coronary artery preparations may be interpreted on the basis of a functional interdependence with pCa(i) changes.