Alkaline pH- and cAMP-induced V-ATPase membrane accumulation is mediated by protein kinase A in epididymal clear cells

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Abstract
In the epididymis, low luminal bicarbonate and acidic pH maintain sperm quiescent during maturation and storage. The vacuolar H+-ATPase (V-ATPase) in epididymal clear cells plays a major role in luminal acidification. We have shown previously that cAMP, luminal alkaline pH, and activation of the bicarbonate-regulated soluble adenylyl cyclase (sAC) induce V-ATPase apical accumulation in these cells, thereby stimulating proton secretion into the epididymal lumen. Here we examined whether protein kinase A (PKA) is involved in this response. Confocal immunofluorescence labeling on rat epididymis perfused in vivo showed that at luminal acidic pH (6.5), V-ATPase was distributed between short apical microvilli and subapical endosomes. The specific PKA activator N6-monobutyryl-3′-5′-cyclic monophosphate (6-MB-cAMP, 1 mM) induced elongation of apical microvilli and accumulation of V-ATPase in these structures. The PKA inhibitor myristoylated-PKI (mPKI, 10 μM) inhibited the apical accumulation of V-ATPase induced by 6-MB-cAMP. Perfusion at pH 6.5 with 8-(4-chlorophenylthio)-2-O-methyl-cAMP (8CPT-2-O-Me-cAMP; 10 μM), an activator of the exchange protein activated by cAMP (Epac), did not induce V-ATPase apical accumulation. When applied at a higher concentration (100 μM), 8CPT-2-O-Me-cAMP induced V-ATPase apical accumulation, but this effect was completely inhibited by mPKI, suggesting crossover effects on the PKA pathway with this compound at high concentrations. Importantly, the physiologically relevant alkaline pH-induced apical V-ATPase accumulation was completely inhibited by pretreatment with mPKI. We conclude that direct stimulation of PKA activity by cAMP is necessary and sufficient for the alkaline pH-induced accumulation of V-ATPase in clear cell apical microvilli.