Mechanism of hyperosmolality stimulation of ANP secretion: its dependency on calcium and sodium

The calcium dependency of hyperosmolality stimulation of atrial natriuretic peptide (ANP) secretion was determined using isolated superfused nonbeating rat left atrium. Increasing osmolality by 65, 85, and 100 mosmol/kgH2O by superfusion with sucrose produced a peak rise in ANP secretion of 1.8-, 2.0-, and 2.7-fold. To determine whether calcium influx played a role in osmolality (osm)-stimulated ANP secretion, atria were superfused with 2 mM lanthanum, a calcium antagonist. Lanthanum inhibited by 85% the response to a 100 mosmol/kgH2O increase in osm. The voltage-dependent calcium channel blocker isradipine had no effect on osm-stimulated ANP secretion, suggesting that calcium influx via voltage-dependent calcium channels was not playing a significant role. Likewise, depleting sarcoplasmic reticulum calcium with 1 microM ryanodine did not block the response to osm, suggesting that calcium influx was not adequate to induce consequential release of calcium from the sarcoplasmic reticulum. To determine whether calcium influx was via Na(+)-Ca2+ exchange, we determined the sodium dependency of osm-stimulated ANP secretion. Replacement of sodium with lithium or choline blocked the secretory response to 100 mosmol/kgH2O. We conclude that osm-stimulated ANP secretion is calcium and sodium dependent. Calcium influx via Na(+)-Ca2+ exchange is highly implicated as the mechanism of cellular calcium entry.

Tetraethylammonium transport by isolated perfused snake renal tubules

Tetraethylammonium (TEA) transport was studied in isolated perfused snake (Thamnophis spp.) proximal renal tubules. Unidirectional lumen-to-bath (J1----bTEA) and bath-to-lumen (J1----bTEA) fluxes exhibited saturation kinetics, but Jb----1TEA also exhibited an apparent diffusive component and J1----bTEA did not. Jb----1TEA exceeded J1----bTEA at all concentrations studied, resulting in net TEA secretion. Transport into cells across both luminal and peritubular membranes was apparently against an electro-chemical gradient and was inhibited by cyanide. Km for J1----bTEA (5.9 microM) was about one-third Km for Jb----1TEA (19.9 microM), indicating greater affinity of the luminal transporter for TEA; but Vmax for Jb----1TEA (153 fmol X min-1 X mm-1) was about six times Vmax for J1----bTEA (27 fmol X min-1 X mm-1), indicating a greater capacity of the peritubular transporter for TEA, which could account for net TEA secretion. Jb----1TEA was inhibited by N-methylnicotinamide (NMN) in the bath, but J1----bTEA was inhibited initially and then apparently transstimulated by NMN in the lumen, indicating possible countertransport. J1----bTEA, but not Jb----1TEA, was significantly reduced by replacement of sodium with sucrose, indicating possible sodium dependency of the luminal transporter. All data indicate active (either primary or secondary) TEA transport at both luminal and peritubular membranes but net transepithelial transport in the bath-to-lumen direction.