The effect of cAMP on ion transport in the proximal tubular cells in bullfrog kidney.

Abstract

Using a direct cellular micropuncture technique with doublebarreled ion-selective microelectrodes, we investigated the effect of dibutyryl-cyclic AMP (db-cAMP) on the membrane potential and the transport of Na +, K +, and H + in doubly-perfused bullfrog proximal tubules. The peritubular membrane potential difference (EM) and the intracellular K+, Na+ activities ((K);, (Na);) or intracellular and luminal pH were monitored continuously after peritubular administration of dbcAMP (10-3-10-4 M). Results: 1) db-cAMP hyperpolarized the EM by 8.0 mV with an increase of (K); by 4.8 mEq/l; 2) the peritubular administration of high K + (13.5 and 50 mM) solutions depolarized the EM by 11.5 and 41.5 mV, respectively. The high K + perfusate with db-cAMP produced a depolarization to the same level as that in simple high K + perfusion without db-cAMP; 3) db-cAMP transiently blocked the luminal acidification concomitantly with a cellular alkalinization by about 0.1 pH; and 4) db-cAMP caused a decrease of (Na); by 5.0 mEq/l. Conclusions: 1) cAMP causes an increase of K+ permeability in the peritubular membrane; 2) cAMP induces a cytosol alkalosis by enhancing incorporation of H+ into the subcellular vesicles, thus favoring the activation of Na+/K+ pump; and 3) cAMP, in effect, suppresses the Na+/H+ exchange mechanism in the luminal membrane and transiently blocks the proximal urinary acidification.

Dibutyryl Cyclic Adenosine Monophosphate Stimulates the Sodium Pump in Rabbit Renal Cortical Tubules

“…We have recently provided support for these results with the finding that the sodium pump activity mea sured electrophysiologically is increased in iso lated collapsed rabbit proximal convoluted tu bules with other cAMP analogues [21]. Our findings may go some way in explaining results obtained in amphibian proximal tubules in which db-cAMP causes a cell hyperpolarization associated with an elevation of intracellu lar potassium and a fall in intracellular sodium [11][12][13], Although it was suggested that these results stemmed primarily from an inhibition of apical sodium entry [11], a component of sodi um pump stimulation might also be involved, as was suggested [ 12], Although an effect of db-cAMP on sodium pump activity was clear in the present study, we were unable to find an effect of the combined addition of forskolin and IB MX on either base line Q 0 2 or Q 0 2 generated by maximal pump stimulation with nystatin. The combination of these drugs would be expected to cause a sus tained elevation in intracellular cAMP via the stimulation of adenylate cyclase by forskolin and an inhibition of phosphodiesterases by IBMX [16], Indeed, 10 4 M forskolin has been shown to niimick the effect of db-cAMP in the stimulation of the sodium pump in rat cortical homogenate [7], and 10 5 M forskolin mimicks the effect of 10 1 M db-cAMP with regard to in hibition of luminal proton secretion in isolalcdperfused rabbit proximal convoluted tubule [I], On the other hand, it is possible that forskolin is unable to elevate cAMP to levels equivalent to those attained by incubation with 10 ' M dbcAMP in intact rabbit proximal cells.…”

“…This demonstrates that, under control conditions, tu bule suspensions were able to maintain intra cellular sodium well below a saturating concen tration for the sodium pump. Effect o f db-cAMP db-cAMP is an agent which has been com monly used to mimick the effects of endoge nously produced cAMP in a variety of proximal tubule preparations, including rabbit proximal tubules perfused in vitro [1,8,9], micropunctured rat proximal tubules [10], micropunctured frog proximal tubules [11][12][13], rat proximal tu bule suspensions [4], rat renal cortical homog enates [7], and proximal tubule brush border membranes [2]. In the present study, in suspen sions o f rabbit cortical tubule suspensions con sisting mainly of proximal tubules, we have found that, while 1 mM db-cAMP causes only a small increase in baseline Q():.…”