Abstract
1. The modulatory effects of carbachol, endothelin-1 and isoprenaline upon Na(+)-H+ exchange were examined in dog cardiac Purkinje fibres. Intracellular pH (pHi) and intracellular sodium activity (aiNa) were recorded using pH and Na(+)-selective microelectrodes. Acid extrusion via Na(+)-H+ exchange was estimated from the pHi recovery rate (multiplied by intrinsic buffering power (beta i) and adding mean background acid load) in response to an internal acid load induced by the removal of 20 mM NH4Cl. All experiments in this work were performed in Hepes-buffered solutions at 37 degrees C. 2. beta i was estimated at various values of pHi in the range of 7.4-6.4 and was calculated from the fall of pHi induced by the addition and removal of NH4Cl. Experiments were performed when Na(+)-H+ exchange was blocked. The values of beta i in this tissue were only slightly dependent on pHi in the range of 7.4-6.4 with an empirical relationship: beta i = -4.69 pHi + 64.59. 3. Endothelin-1 (10(-8) M) alkalinized the resting pHi by approximately 0.1 pH unit and accelerated acid extrusion, by approximately 96%, at pHi approximately 6.9. A reduction of background acid loading within the cell cannot account for the augmentation of pHi recovery, since the rate of acid extrusion was not changed either at resting pHi or at internal acidification in Na(+)-free solution (a measure of background loading) by the addition of endothelin-1. The protein kinase C inhibitors staurosporin (10(-6) M) and 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7, 50 microM) could not block the effect of endothelin-1 on the antiporter. 4. At pHi approximately 6.8, carbachol (7.5 x 10(-4) M) accelerated pHi recovery by approximately 68% and alkalinized the resting pHi by approximately 0.1 pH unit. This stimulatory effect of carbachol was completely blocked by pretreatment with atropine (10(-4) M) and staurosporine 10(-6) M. The background acid load was not reduced by adding carbachol, since the acid extrusion during pHi recovery or at the resting state was not affected by the addition of carbachol to a sodium-free solution. 5. Isoprenaline (10(-6) M) slowed pHi recovery by approximately 45% measured at pHi 6.9 with no change in resting pHi. A rise in background acid loading could not account for the reduction of acid extrusion. Pretreated with atenolol (10(-6) M), a beta 1-selective antagonist, completely blocked the effect of isoprenaline.(ABSTRACT TRUNCATED AT 400 WORDS)