COUPLING OF DUAL ACID EXTRUSION IN THE GUINEA-PIG ISOLATED VENTRICULARY MYOCYTE TO ALPHA-1-ADRENOCEPTORS AND BETA-ADRENOCEPTORS

1. Intracellular pH (pH(i)) was recorded in single, isolated guinea-pig ventricular myocytes using the pH-sensitive fluorophore, carboxy-SNARF-1 (AM-loaded). 2. The dual acid extrusion system in this cell (Na+-H+ antiport and Na+-HCO3- symport) was activated by inducing an intracellular acid load, produced by addition and subsequent removal of extracellular 10 mm NH4Cl. Under these conditions, it is known that both acid-equivalent extruders are activated about equally. 3. Application of phenylephrine (100 mum; alpha-adrenergic agonist) resulted in an inhibition of pH(i) recovery from an acid load, recorded in HCO3--buffered medium containing 1.5 mm amiloride (amiloride inhibits Na+-H+ antiport; under these conditions pH(i) recovery is mediated through only the Na+-HCO3- symport carrier). This inhibitory effect of phenylephrine was prevented by the alpha1-antagonist, prazosin (0.1 mum) and was unaffected by propranolol (1 mum). 4. Application of phenylephrine in Hepes-buffered medium (only Na+-H+ antiport is active under these conditions) elicited a stimulation of pH(i) recovery, again prevented by prazosin (0.1 mum). 5. These results point to an alpha1 inhibition of Na+-HCO3- symport and an alpha1 stimulation of Na+-H+ antiport. 6. Both adrenaline (1-5 mum) and noradrenaline (5 mum) slowed pH(i) recovery recorded in HCO3--buffered solution containing amiloride (1.5 mm). The similarity of this result with that obtained previously using phenylephrine (paragraph 3) suggests that all three agonists inhibit the Na+-HCO3- symport through alpha1 activation. 7. Isoprenaline (1 mum; beta-adrenergic agonist) slowed pH(i) recovery in Hepes-buffered solution but stimulated recovery in a HCO3--buffered solution containing amiloride (1.5 mm). These results suggest that beta activation slows Na+-H+ antiport but stimulates Na+-HCO3- symport. 8. When both acid-equivalent extrusion carriers were inhibited in Na+-free, HCO3--buffered medium, phenylephrine or isoprenaline had no effect on pH(i), ruling out any effect of the adrenergic agonists on background acid-loading mechanisms. 9. Under physiological conditions (CO2/HCO3--buffered solution, no amiloride), when both acid extruders would be activated by an intracellular acid load, application of phenylephrine, adrenaline or noradrenaline were found to slow pH(i) recovery. In contrast, isoprenaline stimulated pH(i) recovery under the same conditions. 10. We conclude that, in the guinea-pig ventricular myocyte, Na+-HCO3- symport and Na+-H+ antiport are oppositely coupled to alpha1-receptors, i.e. the symport is inhibited and the antiport stimulated by receptor activation. In contrast, coupling of the two carriers to beta-receptors is the reverse of that to alpha1-receptors (i.e. beta-activation produces symport stimulation and antiport inhibition). Our results suggest that, in this cell, alpha1 control of the dual extrusion system is the dominant factor determining the response of total acid extrusion to the physiological agonists, adrenaline and noradrenaline.