To date various types of Cl- currents have been recorded in cardiac myocytes from different regions of the heart and from different species. Most of these are silent under basal conditions, but are rapidly activated under the influence of various agonists or physical stress that, in the long term, also lead to development of hypertrophy. Previously, we identified three different Cl- channel activities in neonatal rat cardiomyocytes: (i) Ca2+ regulated, (ii) cAMP regulated (cystic fibrosis transmembrane conductance regulator Cl- channels) and (iii) osmoregulated Cl- channels. In this study, we examined comparatively the effects of cyclic stretch and endothelin-1 (ET-1) on Cl- channel activity in primary cultures of neonatal rat ventricular myocytes using an 125I-efflux assay. About 4min after the start of the 125I-efflux (mean basal rate amounts 6.3% of total 125I incorporated/min), the addition of 10nM ET-1 or the application of cyclic stretch rapidly and transiently increased 125I-efflux by 3.8%/min and 0.8%/min respectively above the basal rate. The stretch induced 125I-efflux rate could be blocked by 100µM Gd3+ but it had no effect on the ET-1 response. After 24h stimulation by ET-1 or cyclic stretch the myocytes responded by hypertrophy which is detected by increases of 3H-leucine incorporation into protein and protein/DNA ratio. In conclusion, cyclic stretch as well as ET rapidly and transiently activate Cl- channels in rat neonatal cardiomyocytes. The results suggest that the activation of distinct types of Cl- channels (co)transduce the stretch- and agonist-induced hypertrophic responses in these myocytes.

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