Parathyroid Hormone Induces a Calmodulin-Dependent Alteration in Phosphodiesterase Activity of Rat Kidney in Vivo*

In the basal state cyclic nucleotide phosphodiesterase in cytosol of rat kidney cortex is characterized by at least two kinetically distinct activities, with Michaelis-Menten constant (Km) for cAMP of 3.3 X 10(-6) and 3.7 X 10(-5) M. Eight minutes after in vivo injection of parathyroid extract, and persisting over 4 h, phosphodiesterase showed only one apparent activity, Km = approximately 1 X 10(-5) M. This change in kinetic profile was mimicked by the ip injection of 10 mg/kg (Bu)2-cAMP, and was reversed by incubation of cytosol with trifluoperazine or EGTA. Incubation of control cytosol with purified calmodulin duplicated the effect of hormone injection. Chromatography of control cytosol over diethylaminoethyl-cellulose resolved two activity peaks. Peak I, corresponding to the high Km activity of whole cytosol, was stimulated by calmodulin, representing a change in maximum velocity but not in substrate affinity. In addition, calmodulin stimulated the hydrolysis by Peak I of 3',5'-cyclic guanosine monophosphate. Peak II activity was not altered by calmodulin. These studies demonstrate that PTH promotes a rapid and sustained alteration in phosphodiesterase kinetics which is mediated by calcium and calmodulin, and which reflects the activation of the high Km component of the enzyme.