Inositol 1,4,5-trisphosphate releases Ca2+from a nonmitochondrial store site in permeabilized rat cortical kidney cells

Inositol 1,4,5-trisphosphate releases Ca2+from a nonmitochondrial store site in permeabilized rat cortical kidney cells. We have recently shown that inositol 1,4,5-trisphosphate (IP3) releases Ca2+from the endoplasmic reticulum of pancreatic acinar cells and suggested that IP3may function as a second messenger of hormonal receptors to mobilize Ca2+from intracellular stores (Streb et al, 1983, Streb et al, 1984). In rat kidney cortical tubules and microdissected mouse proximal tubules, an increased turnover of polyphosphoinositide metabolism following hormonal stimulation with angiotensin II-amide and phenylephrine has been reported (Wirthensohn et al, 1984; Wirthensohn et al, 1985). This suggests that IP3, one of their hydrolysis products, increases during hormonal stimulation. We therefore investigated the effect of angiotensin II-amide and IP3on intracellular Ca2stores in saponin-treated cells and homogenate from rat kidney cortex. Saponin-treated isolated cortical kidney cells or homogenate was incubated in a high K+buffer in the presence of MgATP and respiratory substrates. Ca2+uptake was determined by measuring the free Ca2+concentration of the surrounding medium with a Ca2+specific macroelectrode. Addition of cells or homogenate to the incubation medium resulted in a decrease of the medium free Ca2+concentration until a steady-state concentration of 5.7 ± 0.2 × 10-7mole/1 was obtained. In the presence of mitochondrial inhibitors Ca2+uptake rate was reduced, whereas the steady-state concentration was unchanged. In contrast, in the presence of the Ca2+-ATPase inhibitor vanadate mitochondrial uptake proceeded at the same rate as the control, but the steady-state concentration was higher (6.9 ± 0.2 × 10-7mole/1). When IP3(5 µmole/1) was added to saponin-treated cells or homogenate at steady-state in the presence or absence of mitochondrial inhibitors, Ca2+was released to the medium, followed by Ca2+reuptake to the previous level. Similarly, addition of angiotensin II-amide (10-5mole/1) to saponin-treated cells resulted in Ca2+release. Our data suggest that both a mitochondrial and a nonmitochondrial Ca2+pool are involved in regulation of cytosolic free Ca2+in saponin-treated rat kidney cortex cells. Steady-state Ca2+concentration is determined by the nonmitochondrial Ca2+pool. IP3may function as a second messenger for hormone action to release Ca2+from a nonmitochondrial store in rat cortical kidney cells