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.