DEFECT OF COLD-SENSITIVE RESPONSE IN CALMODULIN MUTANTS OF PARAMECIUMAND THE RESTORATION BY CYCLIC-NUCLEOTIDE

Wild type and calmodulin mutants (cam) of Paramecium tetraurelia were examined for cold-sensitive responses. Among mutants tested, cam(12) a nd cam(13) mutants, which have substitutions in N-terminal lobe of cal modulin molecule, reduced both responses in the swimming and the membr ane potential. Under voltage clamp conditions, the cooling stimulus to the wild type cell induced a transient inward current whose amplitude increased with the rate of temperature drop. The cam(12) cell did not induce inward currents in response to cooling with a rate slower than -0.4 degrees C/s. The reduced current response of cam(12) mutant was restored by an external application of a phosphodiesterase inhibitor, theophylline. Also, an intracellular injection of hydrolysis-resistant cyclic nucleotides, either 8-bromoadenosine 3',5'-cyclic monophosphat e (8-Br-cAMP) or 8-bromoguanosine 3,5'-cyclic monophosphate (8-Br-cGMP ), restored the current response. Such restoration was accompanied by shifts of the resting potential to hyperpolarized levels and by an inc rease in the membrane conductance. The results suggest the possibility that calmodulin and cyclic nucleotide regulate K+ channels responsive to the cooling stimulus.