ANF inhibits vasopressin-induced Ca2+ mobilization and contraction in glomerular mesangial cells

It has been suggested that atrial natriuretic factor (ANF) may increase glomerular filtration rate (GFR) via alteration of the glomerular ultrafiltration area, which is at least in part regulated by the contractile state of mesangial cells. The present study therefore investigated the effect of ANF (atriopeptin III) on arginine vasopressin (AVP)-induced Ca2+ mobilization and contraction in cultured rat mesangial cells. AVP (10(-8) M) increased intracellular Ca2+ ([ Ca2+]i) as measured by the quin2 method (142.1 +/- 3.7 vs. 297.9 +/- 8.9 nM, P less than 0.001). This effect, however, was diminished by 10(-8) M ANF (297.9 +/- 8.9 vs. 233.1 +/- 9.3 nM, P less than 0.0025). ANF alone did not affect basal [Ca2+]i. AVP-stimulated 45Ca2+ efflux (cpm.mg prot-1.30 s-1) from mesangial cells within 30 s (1,432 +/- 70 basal vs. 2,751 +/- 287 AVP at 30 s, P less than 0.001). This effect of AVP on 45Ca2+ efflux was also inhibited by ANF (1,403 +/- 62 basal vs. 1,584 +/- 87 AVP and ANF, NS); ANF alone had no effect on spontaneous 45Ca2+ efflux. Furthermore, ANF inhibited AVP-induced 45Ca2+ uptake, which is purportedly one of the major determinants of the sustained phase of cell contraction. Cell contraction was assessed by digital imaging analysis. AVP (10(-7) M) caused contraction of 33.8 +/- 2.0% of the cells, but the contractile response decreased to 20.7 +/- 2.3% in the presence of 10(-9) M ANF and was virtually abolished in the presence of 10(-8) M ANF. Guanosine 3',5'-cyclic monophosphate (cGMP) has been proposed as the second messenger for ANF. ANF (10(-7) M) stimulated cGMP production significantly (0.54 +/- 0.12 vs. 22.30 +/- 5.91 pmol.mg prot-1.10 min-1, P less than 0.01), but it had no effect on cGMP at concentrations of 10(-8) M and below. 8-Bromo-cGMP (8-BrcGMP) attenuated the stimulatory effect of AVP on [Ca2+]i, 45Ca2+ efflux, and 45Ca2+ uptake. AVP-induced cell contraction was also decreased in the presence of 2 mM 8-BrcGMP (33.8 +/- 2.0% vs. 18.8 +/- 2.6%, P less than 0.005). The present results suggest that ANF may increase GFR in part by blocking the cellular action of AVP and possibly other vasoconstrictors on glomerular mesangial cells, thus affecting glomerular filtration area. A role of cGMP as second messenger for ANF is possible, but it appears unlikely to be the only mediator for the effects of ANF.