CYCLIC-NUCLEOTIDES ALTER INTRACELLULAR FREE MG2+ IN RENAL EPITHELIAL-CELLS

Intracellular Mg2+ plays an important role in cell physiology. Studies were performed on MDCK cells and primary cortical thick ascending limb (CTAL) cells to determine hormonal influences on intracellular Mg2+ control. Free Mg2+ ([Mg2+]i) was measured by fluorescence with mag-fura-2. Addition of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcG-MP, 10(-4) M) to subconfluent MDCK cells resulted in rapid increases in [Mg2+]i from basal levels of 552 +/- 6-mu-M to peak concentrations of 682 +/- 5-mu-M, whereas 8-BrcAMP (10(-4) M) led to significant decreases in [Mg2+]i from 538 +/- 5 to 362 +/- 17-mu-M. These effects of cyclic nucleotides were dose dependent with half-maximal concentrations (EC50) of approximately 10(-5) M for both increments in [Mg2+]i with cGMP and decrements in [Mg2+]i with cAMP. Atrial natriuretic peptide (ANP) and cGMP increased Mg2+ in porcine primary CTAL cells from 525 +/- 12 to 592 +/- 18-mu-M and from 538 +/- 8 to 609 +/- 18-mu-M, respectively. The increment in [Mg2+]i with ANP was dose responsive with EC50 values of approximately 10(-11) M suggesting that these effects may be of physiological importance. Parathyroid hormone and calcitonin and their second messenger, cAMP, diminished Mg2+ by approximately 80-mu-M. The EC50 value for calcitonin was in the order of 10(-9) M. The changes in [Mg2+]i, whether increases with ANP or cGMP and decreases with PTH, calcitonin, or cAMP, were rapid in nature and independent of changes in intracellular free Ca2+ concentration. These data indicate that [Mg2+]i is influenced by peptide hormones and their second messengers likely through activation of appropriate protein kinases. The functional role of these changes remains to be determined.