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-pi g ventricular myocytes using the pH-sensitive fluorophore, carboxy-SNA RF-1 (AM-loaded). 2. The dual acid extrusion system in this cell (Na+- H+ antiport and Na+-HCO3- symport) was activated by inducing an intrac ellular acid load, produced by addition and subsequent removal of extr acellular 10 mm NH4Cl. Under these conditions, it is known that both a cid-equivalent extruders are activated about equally. 3. Application o f phenylephrine (100 mum; alpha-adrenergic agonist) resulted in an inh ibition of pH(i) recovery from an acid load, recorded in HCO3--buffere d medium containing 1.5 mm amiloride (amiloride inhibits Na+-H+ antipo rt; under these conditions pH(i) recovery is mediated through only the Na+-HCO3- symport carrier). This inhibitory effect of phenylephrine w as prevented by the alpha1-antagonist, prazosin (0.1 mum) and was unaf fected by propranolol (1 mum). 4. Application of phenylephrine in Hepe s-buffered medium (only Na+-H+ antiport is active under these conditio ns) elicited a stimulation of pH(i) recovery, again prevented by prazo sin (0.1 mum). 5. These results point to an alpha1 inhibition of Na+-H CO3- symport and an alpha1 stimulation of Na+-H+ antiport. 6. Both adr enaline (1-5 mum) and noradrenaline (5 mum) slowed pH(i) recovery reco rded in HCO3--buffered solution containing amiloride (1.5 mm). The sim ilarity of this result with that obtained previously using phenylephri ne (paragraph 3) suggests that all three agonists inhibit the Na+-HCO3 - symport through alpha1 activation. 7. Isoprenaline (1 mum; beta-adre nergic agonist) slowed pH(i) recovery in Hepes-buffered solution but s timulated recovery in a HCO3--buffered solution containing amiloride ( 1.5 mm). These results suggest that beta activation slows Na+-H+ antip ort but stimulates Na+-HCO3- symport. 8. When both acid-equivalent ext rusion carriers were inhibited in Na+-free, HCO3--buffered medium, phe nylephrine or isoprenaline had no effect on pH(i), ruling out any effe ct of the adrenergic agonists on background acid-loading mechanisms. 9 . Under physiological conditions (CO2/HCO3--buffered solution, no amil oride), when both acid extruders would be activated by an intracellula r acid load, application of phenylephrine, adrenaline or noradrenaline were found to slow pH(i) recovery. In contrast, isoprenaline stimulat ed pH(i) recovery under the same conditions. 10. We conclude that, in the guinea-pig ventricular myocyte, Na+-HCO3- symport and Na+-H+ antip ort are oppositely coupled to alpha1-receptors, i.e. the symport is in hibited and the antiport stimulated by receptor activation. In contras t, coupling of the two carriers to beta-receptors is the reverse of th at to alpha1-receptors (i.e. beta-activation produces symport stimulat ion and antiport inhibition). Our results suggest that, in this cell, alpha1 control of the dual extrusion system is the dominant factor det ermining the response of total acid extrusion to the physiological ago nists, adrenaline and noradrenaline.