Regulation of Renal Na-HCO₃ Cotransporter: VIII. Mechanism of Stimulatory Effect of Respiratory Acidosis

Abstract.

We examined the effect of respiratory acidosis on the Na-HCO₃ cotransporter activity in primary cultures of the proximal tubule of the rabbit exposed to 10% CO₂ for 5 min, 2, 4, 24 and 48 hr. Cells exposed to 10% CO₂ showed a significant increase in Na-HCO₃ cotransporter activity (expressed as % of control levels, 5 min: 142 ± 6, 2 hr: 144 ± 13, 4 hr: 145 ± 11, 24 hr: 150 ± 15, 48 hr: 162 ± 24). The increase in activity was reversible after 48 hr. The role of protein kinase C (PKC) on the stimulatory effect of respiratory acidosis on the cotransporter was examined in presence of PKC inhibitor calphostin C or in presence of PKC depletion. Both calphostin C and PKC depletion prevented the effect of 10% CO₂ for 5 min or 4 hr to increase the activity of the cotransporter. 10% CO₂ for 5 min or 4 hr increased total and particulate fraction PKC activity. To examine the role of phosphotyrosine kinase (PTK) on the increase in cotransporter activity we studied the effect of two different inhibitors, 2-hydroxy-5-(2,5-dihydroxylbenzyl) aminobenzoic acid (HAC) and methyl 2,5-dihydroxycinnamate (DHC) which inhibit phosphotyrosine kinase in basolateral membranes. Cells were pretreated either with vehicle or HAC or DHC and then exposed to 10% CO₂ for 5 min or 4 hr. In cells treated with vehicle, 10% CO₂ significantly increased cotransporter activity as compared to control cells exposed to 5% CO₂. This stimulation by 10% CO₂ was completely prevented by HAC or DHC at 5 min (5% CO₂: 1.8 ± 0.2, 10% CO₂: 2.6 ± 0.2, 10% CO₂+ HAC: 1.6 ± 0.2, 10% CO₂: +DHC: 2.0 ± 0.3 pH unit/min) and also at 4 hr. The protein synthesis inhibitors actinomycin D and cycloheximide appear to prevent the effect of 10% CO₂ for 4 hr on the cotransporter. Our results show that early respiratory acidosis stimulates the Na-HCO₃ cotransporter through PKC and PTK-dependent mechanisms and the late effect appears to be mediated through protein synthesis.