Effects of calcitonin on cytosolic Ca in a suspension of rabbit medullary thick ascending limb tubules.

A basal level of cytosolic free Ca (Caf) of 112 +/- 7 nM (n = 15) was measured in rabbit medullary thick ascending limb tubules that were loaded with the fluorescent Ca indicator quin 2. Calcitonin addition (10(-6) M) caused a greater than twofold increase in Caf. The KD for calcitonin stimulation of Caf was 8 X 10(-11) M. The rise in Caf had a half time of approximately 1.5 min. Neither parathyroid hormone (10(-6) M), dibutyryladenosine 3',5'-cyclic monophosphate (10(-4) M), nor forskolin (2 X 10(-5) M) had any significant effect on Caf levels, consistent with the rise in Caf being independent of an elevation in adenosine 3',5'-cyclic monophosphate. The effects of ouabain and the mitochondrial uncoupler, 1799, were also tested to obtain further information regarding the regulation of Caf. Ouabain (10(-4) M) caused no significant elevation in Caf; in contrast, 15 min of incubation with 1799 (10(-5) M) produced a twofold increase in Caf.