STRETCH-ACTIVATED ION CHANNELS IN TISSUE-CULTURED CHICK HEART

With use of single-channel patch-clamp recording, we found five distinct types of stretch-activated ion channels (SACs) in tissue-cultured embryonic chick cardiac myocytes. With 140 mM K+ saline in the pipette, four channels had linear conductances of congruent-to 25, 50, 100, and 200 pS and the other channel was an inward rectifier of congruent-to 25 pS at 0 mV membrane potential. The 100- and 200-pS channels were K+ selective, whereas the others passed alkali cations and Ca2+. From reversal potentials, the permeability ratio of K+/Na+, P(K)/P(Na), was 3-7 for nonselective channels and 7-16 for K+-selective channels. Channel density was congruent-to 0.3/mum2 for linear conductances and congruent-to 0.1/mum2 for inward rectifier. Open-channel noise was a function of pipette filling solution with root-mean-square (RMS) noise increasing in the order K+ < isosmotic sucrose (plus trace ions) < Na+, probably reflecting short-lived block by extracellular ions. All channels were blocked by 20 muM Gd3+. The 25-pS linear channel was also blocked by 12.5 muM tetrodotoxin and 10 muM diltiazem, but the others were insensitive at these concentrations. Extracellular Cs+ and tetraethylammonium chloride did not block any channels. We saw no SAC activity in cells grown without embryo extract (EE), which demonstrates that channel expression, or some necessary cofactor, is under control of growth factors. Basic fibroblast growth factor (FGF) could replace EE in supporting channel expression. The presence of SACs capable of generating inward currents might explain how stretch increases automaticity in the heart. Because some SACs were permeable to Ca2+, they could contribute to the Starling curve and perhaps to initiating stretch-induced hypertrophy.