Direct action of aldosterone on bicarbonate reabsorption in in vivo cortical proximal tubule

Abstract

The direct action of aldosterone (10(-12) M) on net bicarbonate reabsorption (J(HCO(3)(-))) was evaluated by stationary microperfusion of an in vivo middle proximal tubule (S2) of rat kidney, using H ion-sensitive microelectrodes. Aldosterone in luminally perfused tubules caused a significant increase in J(HCO(3)(-)) from a mean control value of 2.84 +/- 0.08 [49/19 (n degrees of measurements/n degrees of tubules)] to 4.20 +/- 0.15 nmol.cm(-2).s(-1) (58/10). Aldosterone perfused into peritubular capillaries also increased J(HCO(3)(-)), compared with basal levels during intact capillary perfusion with blood. In addition, in isolated perfused tubules aldosterone causes a transient increase of cytosolic free calcium ([Ca(2+)](i)), monitored fluorometrically. In the presence of ethanol (in similar concentration used to prepare the hormonal solution), spironolactone (10(-6) M, a mineralocorticoid receptor antagonist), actinomycin D (10(-6) M, an inhibitor of gene transcription), or cycloheximide (40 mM, an inhibitor of protein synthesis), the J(HCO(3)(-)) and the [Ca(2+)](i) were not different from the control value; these drugs also did not prevent the stimulatory effect of aldosterone on J(HCO(3)(-)) and on [Ca(2+)](i). However, in the presence of RU 486 alone [10(-6) M, a classic glucocorticoid receptor (GR) antagonist], a significant decrease on J(HCO(3)(-)) and on [Ca(2+)](i) was observed; this antagonist also inhibited the stimulatory effect of aldosterone on J(HCO(3)(-)) and on [Ca(2+)](i). These studies indicate that luminal or peritubular aldosterone (10(-12) M) has a direct nongenomic stimulatory effect on J(HCO(3)(-)) and on [Ca(2+)](i) in proximal tubule and that probably GR participates in this process. The data also indicate that endogenous aldosterone stimulates J(HCO(3)(-)) in middle proximal tubule.

Genomic and nongenomic stimulatory effect of aldosterone on H+-ATPase in proximal S3 segments

“…The proximal tubule is responsible for the reabsorption of 80% of the filtered bicarbonate, a process that depends on the secretion of H ϩ ions by two major mechanisms: the Na ϩ /H ϩ exchanger and H ϩ -ATPase (4,35,39). Recently, studies from our laboratory demonstrated genomic and nongenomic effects of aldosterone on the Na ϩ /H ϩ exchanger in in vivo S2 (25) and in isolated S3 (19) segments of proximal tubule. However, the action of this hormone on H ϩ -ATPase in the proximal tubule is not known.…”

“…However, the receptor involved in the rapid responses to aldosterone is unknown and has been the theme of discussions (13); recently, data from our laboratory suggest that the glucocorticoid receptor (GR) can participate in nongenomic effects of aldosterone on the proximal tubule (19,25). A role for aldosterone in regulating calcium homeostasis is uncertain, but a rise in intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) is a component of several effects of aldosterone, as a second messenger on the hormonal nongenomic effects and as a prerequisite for the genomic action (3,13,19,25,29,36,38).…”

ANG II, ANG-(1-7), ALDO and AVP biphasic effects on Na+/H+ transport: the role of cellular calcium

“…Apical Na + channel activity is the limiting step in the transport process, and it is likely that ALDO ultimately acts to increase the open time of existing ion channels and/ or increase the total number of such channels [225]. However, other protein targets have also been identified, including i) a luminal NHE3 exchanger in the colon [226,227] and the proximal tubule [228] and a basolateral NHE1 in the proximal tubule [229,230], ii) an Na + /K + -ATPase in human kidney proximal tubule (HKC11) cells [231], iii) a luminal thiazide-sensitive Na + /Cl − cotransporter in the distal renal tubule that appears to mediate Na+ reabsorption in response to volume depletion [232], and iv) a renal proximal H + -ATPase [233].…”

“…The physiological and clinical relevance supporting these rapid effects remains unclear, but their existence has been described in various target organs and cells, including amphibian skin and urinary bladders [258,259], vascular smooth muscle cells and endothelial cells [260], skeletal muscle cells [261], human mononuclear leukocytes [262], cardiac myocytes [263], skin fibroblasts from MR-knockout mice [256], colonic epithelial cells [264] and isolated colonic crypts [265]. Several sites in the kidney, particularly cultured kidney cells, have been shown to be sensitive to non-genomic ALDO action [266], including principal cells that were freshly isolated from rabbits [267], the human distal colon [268], in vivo renal proximal tubules (S2 segment) [228], isolated renal proximal tubules (S3 segment) [229,233], medullary thick ascending limbs [269] and renal collecting duct cells [270]. Its non-genomic actions include effects on signal transduction pathways and ion transporters, such as the epithelial Na + channel [267], the Na + /H + exchanger [228,229,271,272] and the vacuolar H + -ATPase [230,233,258,259,273].…”

“…Several sites in the kidney, particularly cultured kidney cells, have been shown to be sensitive to non-genomic ALDO action [266], including principal cells that were freshly isolated from rabbits [267], the human distal colon [268], in vivo renal proximal tubules (S2 segment) [228], isolated renal proximal tubules (S3 segment) [229,233], medullary thick ascending limbs [269] and renal collecting duct cells [270]. Its non-genomic actions include effects on signal transduction pathways and ion transporters, such as the epithelial Na + channel [267], the Na + /H + exchanger [228,229,271,272] and the vacuolar H + -ATPase [230,233,258,259,273].…”