Pharmacological analysis of the transmembrane action potential configuration in myoepithelial cells of the spontaneously beating heart of the ascidian Styela rustica in vitro.

The mechanisms of action potential (AP) generation in the myoepithelial cells of the tunicate heart are not yet well understood. Here, an attempt was made to elucidate these mechanisms by analyzing the effects of specific blockers of K+, Na+ and Ca2+ currents on the configuration of transmembrane APs and their frequency in the spontaneously beating ascidian heart. In addition, an immunocytochemical analysis of heart myoepithelial cells was performed. Staining with anti-FMRF-amide and anti-tubulin antibodies did not reveal any nerve elements within the heart tube. Treatment with 1 mmol l-1 TEA (IK blocker) resulted in depolarization of heart cell sarcolemma by 10 mV, and inhibition of APs generation was recorded after 3 min of exposure. Prior to this moment, the frequency of AP generation in a burst decreased from 16-18 to 2 beats min-1 owing to prolongation of the diastole. After application of ivabradine (3 or 10 µmol l-1), the spontaneous APs generation frequency decreased by 24%. Based on these results and published data, it is concluded that the key role in the automaticity of the ascidian heart is played by the outward K+ currents, Na+ currents, activated hyperpolarization current If and a current of unknown nature IX.