Abstract
Effects of parathyroid hormone (PTH) and the divalent cation ionophore A-23187 on concentrations of cyclic AMP (cAMP) and cyclic GMP (cGMP) were investigated in isolated renal tubules from male hamsters. Both PTH and A-23187 increased cGMP concentrations, effects which required the presence of extracellular calcium. Increases in cGMP in response to both agents were proportionate to concentrations of calcium between 0.01 and 1.0 mM.Pretreatment of tubules with ionophore prevented any further increase in cGMP in response to PTH. Increases in cAMP in response to PTH were not altered by extracellular calcium, while smaller increases due to A-23187 were completely calcium-dependent. A-23187 increased efflux of 45Ca from renal tubules and resulted in decreased accumulation and net uptake of 45Ca at low extracellular calcium concentrations (2–3 μM). Increasing extracellular calcium (0.1–3 mM) proportionately reversed the inhibition in 45Ca accumulation and resulted in increases in cGMP levels which were directly related to increases in 45Ca accumulation. The results indicate that changes in cGMP concentrations of isolated renal tubules following either PTH or A-23187 are related to changes in intracellular calcium.
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