The role of the DHP receptor in excitation-contraction coupling in the heart muscle of Ciona intestinalis (Tunicata : ascidia)

The excitation-contraction (E-C) coupling mechanism and the roles of DHP re ceptors were studied in the heart muscle of the ascidian tunicate, Ciona in testinalis. Spontaneous contractions were blocked by 10 mu M nifedipine, a dihydropyridine (DHP) derivative as well as by EGTA, suggesting the role of the DHP receptor as an L-type Ca2+ channel in normal conditions. On the ot her hand, even in the absence of external Ca2+, in the presence of SCN- or caffeine, contractions were evoked by electrical stimuli and were blocked b y 10 mu M nifedipine. This finding suggests that SCN- and caffeine reveal a normally inactive depolarisation-induced Ca2+ release mechanism involving the DHP receptor in its 'voltage sensor' role. (C) 1999 Elsevier Science B. V. All rights reserved.