Divalent cations modulate PTH-dependent 3',5'-cyclic adenosine monophosphate production in renal proximal tubular cells.
The kidney and parathyroid gland play key roles in calcium (Ca++) homeostasis. Recent data suggest that the kidney, in addition to being a primary target for PTH, also recognizes changes in the concentration of extracellular Ca++, thereby modulating hormone-dependent cAMP production, 1,25-dihydroxyvitamin D synthesis, and renin secretion. In this study, we examined: 1) the effects of varying concentration of divalent cations on PTH-dependent cAMP production in renal proximal tubular cells; and 2) the mechanisms by which extracellular Ca++ exerts its inhibitory effects on cAMP production. Single cell suspensions composed of 80-90% proximal tubular cells were prepared from cortical homogenates by collagenase digestion and sieving. In the presence of 1 mM isobutylmethylxanthine, cAMP content was measured by RIA in 5-15 min incubations and showed a 5- to 6-fold increase in response to PTH (10(-11) -10(-6) M). Increasing extracellular Ca++ and magnesium (Mg++) from 0 and 0.5 mM, respectively, to 5.0 mM inhibited PTH-dependent (3 x 10(-9) M) cAMP production by 54 +/- 4% and 47 +/- 6%, respectively. The half maximal inhibitory concentration for both Ca++ and Mg++ was 0.9 mM. In addition, increasing extracellular barium (Ba++) or strontium (Sr++) from 0-10 mM inhibited PTH-dependent (3 x 10(-9) M) production by 54 +/- 7% and 62 +/- 6% with half of the maximal observed inhibition at 2.2 and 2.7 mM, respectively. The inhibition of PTH-dependent cAMP production by 2.5 mM Ca++ was not reversed by the calcium channel blockers diltiazem or verapamil (10(-4) M). However, changes in intracellular calcium may play some role in the inhibitory effects of Ca++ on cAMP production, since ionomycin (10(-6)-10(-5) M) lowered PTH-dependent cAMP production by 25-36%. Our data suggest that the proximal tubular cell can sense physiologically relevant changes in Ca++, providing a potential mechanism for the modulation of 1,25-dihydroxyvitamin D production or other tubular functions relevant to fluid and mineral homeostasis.
Mathias
R S
RS
Division of Nephrology, University of California, San Francisco 94143.
Brown
E M
EM
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D005260:Female; D015759:Ionomycin / Q000494:pharmacology; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D010281:Parathyroid Hormone / Q000502:physiology*; D051381:Rats; D013997:Time Factors
D002121:Calcium Channel Blockers; D002413:Cations, Divalent; D010281:Parathyroid Hormone; D015759:Ionomycin; D000242:Cyclic AMP