Atrial natriuretic peptide modulates the effect of angiotensin II on the concentration of free calcium in the cytosol of Mandin-Darby canine kidney cells

The effect of angiotensin II (ANG II) and atrial natriuretic peptide (ANP) on intracellular free calcium concentration Ca2+]i was investigated in Mandin-Darby canine kidney (MDCK) cells in culture. Changes in Ca2+]i were monitored fluorometrically with the Ca(2+)-sensitive probe fura-2/AM at 37 degrees C using a Perkin-Elmer LS-5 spectrofluorimeter (excitation 340/380 nm, slit 3 nm; emission 520 nm, slit 10 nm). MDCK cells exhibited a mean baseline Ca2+]i of 98 +/- 10 nM. The addition of increasing concentrations of ANG II (1 pM to 1 microM) to the cell suspension led to a progressive increase in Ca2+]i to 2-3 times basal levels. In contrast, addition of 1 microM ANP to the cell suspension led to a very rapid 60% decrease in Ca2+]i. The addition of 1 pM to 1 microM ANG II immediately after 1 microM ANP caused an increase in Ca2+]i which never exceeded the basal level in the absence of ANP. The data indicate that ANG II increases cell Ca2+]i, as expected, and provide the new observation that ANP reduces Ca2+]i in these cells. Furthermore, ANP reduces the increase in Ca2+]i elicited by ANG II, thus modulating the effect of ANG II on Ca2+]i.