ALPHA-2-ADRENERGIC RECEPTORS REGULATE NA+-H+ EXCHANGE VIA A CAMP-DEPENDENT MECHANISM

The mechanism by which alpha-2-adrenergic receptors regulate Na+-H+ exchange activity in opossum kidney (OK) cells was studied. Because receptors linked to inhibition of adenylate cyclase, like alpha-2-receptors, also can interact with additional signaling mechanisms, we examined specifically the role of adenosine 3',5'-cyclic monophosphate (cAMP) in the signaling pathway controlling Na+-H+ activity in OK cells. Parathyroid hormone (PTH), prostaglandin (PGE1), and forskolin, agents that stimulate cAMP production in these cells, inhibited the rate of amiloride-sensitive Na-22+ uptake by up to 40%. Epinephrine and UK 14304, acting through alpha-2-receptors, were able to reverse this inhibition of Na-22+ uptake to near-control levels and also attenuate PTH-, PGE1-, and forskolin-stimulated cAMP accumulation. Likewise serotonin (5-HT) and SDZ21-009, acting through 5-HT1b receptors, could reverse inhibition of Na-22+ uptake and also attenuate stimulated cAMP accumulation. Neither epinephrine nor serotonin affected the rate of uninhibited Na-22+ uptake. Pertussis toxin pretreatment abolished the effects of alpha-2- and 5-HT1b receptors on both cAMP accumulation and Na-22+ uptake, suggesting that receptor-mediated inhibition of cAMP accumulation is involved in receptor modulation of Na+-H+ exchange activity. In contrast, epinephrine was not able to alter the inhibition of Na-22+ uptake mediated by the membrane-permeant cAMP analogues 8-bromo-cAMP and dibutyryl cAMP at any concentration of analogue that significantly inhibited Na-22+ uptake. These data indicate that alpha-2- as well as 5-HT1b receptors cannot modulate Na+-H+ exchange at steps distal to cAMP production and thus provide strong evidence that receptors that attenuate cAMP accumulation also regulate Na+-H+ exchange in OK cells by a mechanism that relies on their ability to inhibit cAMP production.