Dietary K intake regulates the response of apical K channels to adenosine in the thick ascending limb

Abstract

We used the patch-clamp technique to study the effect of adenosine on the apical 70-pS K channel in the thick ascending limb (TAL) of the rat kidney. Application of 1 M cyclohexyladenosine (CHA), an adenosine analog, stimulated apical 70-pS K channel activity and increased the product of channel open probability and channel number (NP o) from 0.34 to 0.7. Also, addition of CGS-21680, a specific A 2a adenosine receptor agonist, mimicked the effect of CHA and increased NP o from 0.33 to 0.77. The stimulatory effect of CHA and CGS-21680 was completely blocked by H89, an inhibitor of protein kinase A (PKA), or by inhibition of adenylate cyclase with SQ-22536. This suggests that the stimulatory effect of adenosine analogs is mediated by a PKA-dependent pathway. The effect of adenosine analog was almost absent in the TAL from rats on a K-deficient (KD) diet for 7 days. Application of DDMS, an agent that inhibits cytochrome P-450 hydrolase, not only significantly increased the activity of the 70-pS K channel but also restored the stimulatory effect of CHA on the 70-pS K channel in the TAL from rats on a KD diet. Also, the effect of CHA was absent in the presence of 20-HETE. Inhibition of PKA blocked the stimulatory effect of CHA on the apical 70-pS K channel in the presence of DDMS in the TAL from rats on a KD diet. We conclude that stimulation of adenosine receptor increases the apical 70-pS K channel activity via a PKA-dependent pathway and that the effect of adenosine on the apical 70-pS K channel is suppressed by low-K intake. Moreover, the diminished response to adenosine is the result of increase in 20-HETE formation, which inhibits the cAMP-dependent pathway in the TAL from rats on a KD diet. adenosine receptor; adenosine 3Ј,5Ј-cyclic monophosphate; protein kinase A; cytochrome P-450 hydroxylase; arachidonic acid; 20-hydroxyeicosatetraenoic acid ADENOSINE IS GENERATED FROM the hydrolysis of ATP via

Stimulation of A2a adenosine receptor abolishes the inhibitory effect of arachidonic acid on the basolateral 50-pS K channel in the thick ascending limb

“…Our previous study demonstrated that stimulation of adenosine A 2a receptor enhanced the apical and basolateral K channel activity in the TAL (14,20). We speculate that stimulation of adenosine A 2a receptor may serve as a local regulatory factor that links the activity of apical Na-K-Cl cotransporter to the basolateral Cl transport.…”

“…Such a cross-talk has also been observed in regulating the apical K channels in the TAL, which was inhibited by 20-HETE and stimulated by a cAMP-dependent pathway (12,20). However, 20-HETE plays a predominant role in regulating apical K channels since stimulation of adenosine A 2a receptor failed to activate the apical K channels when CYP-omegahydroxylase-dependent AA metabolism was active such as during K depletion (20).…”

Adenosine stimulates the basolateral 50 pS K channels in the thick ascending limb of the rat kidney

“…We have previously shown that adenosine stimulates the apical 70 pS K channels (15), which play an important role in K recycling across the apical membrane. Because K recycling is essential for maintaining the normal function of the Na/K/Cl cotransporter, adenosine should play a role in the regulation of Na transport in the TAL.…”

“…Previous study has also shown that adenosine stimulated the apical 70 pS K channels in the TAL via the A2 adenosine receptor (15). Since both A1 and A2 adenosine receptors have been shown to be expressed in the TAL (28,30) and possibly also in the basolateral membrane, it is conceivable that adenosine may regulate the basolateral K channels.…”

Adenosine and Kidney Function

“…Micropuncture experiments in knockout mice provided further evidence for a tonic, adenosine A 1 receptor-mediated inhibition of Na ϩ reabsorption in a water-impermeable segment of the loop of Henle, possibly the TAL (348). On the other hand, there is evidence in rat TAL that activation of adenosine A 2a receptors can stimulate apical 70-pS K ϩ channel activity via a PKA-dependent pathway (195), but the physiological relevance of these findings remains to be determined. In summary, the available evidence supports the concept that 1) adenosine is released into the renal medulla by medullary TALs in dependence of transport activity and oxygen supply, and 2) the released adenosine via activation of adenosine A 1 receptors inhibits NaCl absorption in the same nephron segments.…”