PHOSPHODIESTERASE ISOENZYMES IN CELL-EXTRACTS OF CULTURED CARROT

Constitutive phosphodiesterase (PDE) activity found in cultured carrot cells did not depend on either Ca2+ or calmodulin (CAM), however, a C AM-dependent isoform of PDE (CAM-PDE) was induced in the cells by the addition of dibutyryl cAMP to the culture which elevates both cAMP and Ca2+ levels of cytosol. Induction of CAM-PDE activity in dibutyryl cA MP-treated carrot cells was markedly inhibited in the presence of a Ca 2+ channel blocker, verapamil, and addition of Ca2+-ionophore A23187 i nto the cell culture resulted in the induction of CAM-PDE. These obser vations suggest that increased Ca2+ but not cAMP in the stimulated car rot cells triggers the induction of the PDE isoenzyme. Affinity of CAM -PDE to the substrate was quite low as compared with the constitutive PDE (K-m values, 0.14 and 0.07 mu M, respectively), however, V of the induced PDE was ca 2.7-fold higher than the constitutive isoenzyme. Th ese results suggest that the constitutive PDE plays an important role in the maintenance of the resting state of carrot cells by keeping cel lular cAMP and Ca2+ levels very low, while CAM-PDE induced in the exci ted cells hydrolyses the messenger nucleotide rapidly under the condit ions of high cAMP and Ca2+ in vivo, as one of the response-decay mecha nisms.