Effect of hypothermia on intracellular Ca2+ in rabbit renal tubules suspended in UW-gluconate preservation solution.

Altered cellular calcium (Ca) homeostasis may be important in mediating hypothermic injury in preserved kidneys. In this study the effect of hypothermic (5 degrees C) storage on ionized intracellular Ca concentration ([Ca]i) in rabbit tubules was examined using Indo-1. Tubules were stored up to 250 min in UW-gluconate solution containing either 0.0, 0.5, 1.5, or 5.0 mM Ca (yielding about 3.6, 62, 371, and 1,010 microM ionized solution Ca (Ca2+) at 5 degrees C, respectively). [Ca]i increased to about 1,600 nM within 1 min after suspension in UW solution followed by a decrease in [Ca]i during the subsequent 60 min in all groups, suggesting mitochondrial Ca sequestration. Thereafter, [Ca]i either 1) increased in tubules incubated with 1.5 and 5.0 mM Ca to levels greater than 2,500 nM; 2) decreased to about 800 nM in tubules incubated with 0.5 mM Ca and then remained stable; or 3) continued to decrease in tubules incubated with 0.0 mM added Ca to reach an apparent steady-state concentration of about 175 nM after 180 min of incubation. The early spike in [Ca]i was unaffected by adding EGTA (solution Ca2+ = 50 nM). Ryanodine eliminated the [Ca]i spike, indicating that cooling in UW-gluconate solution caused release of endoplasmic reticulum Ca. This study shows that [Ca]i initially increases after exposure to UW-gluconate solution and appears to be transiently buffered through intracellular, probably mitochondrial, sequestration. Saturation of cellular buffer mechanisms resulted in a sustained dependence of [Ca]i on extracellular Ca2+. These results support the hypothesis that the effect of Ca on kidney viability is related to solution-induced alterations in [Ca]i.