On the importance of external calcium for sinus node automaticity in spontaneously beating guinea-pig atria
Knorr, A.; Wagner, J.; Schümann, H.J.
Archivos de Farmacologia y Toxicologia 5(2): 79-86
1979
ISSN/ISBN: 0304-8616 PMID: 533320 Document Number: 148692
The influence of Ca2+ on the heart rate of spontaneously beating isolated guinea pig atria was tested. Increasing concentrations of Ca2+ from 0.45-14.4 mM caused a positive chronotropic effect. The maximum increase of frequency [69%] was obtained at 7.2 mM. The positive chronotropic effect of Ca2+ was inhibited by 10-7 M carbachol, a drug known to abbreviate the duration of action potential by an elevation of K efflux up to a concentration of 1.8 mM while 2 .times. 10-7 M also inhibited the effect of Ca2+ up to 14.4 mM. Isoprenaline yielded a synergism in action together with Ca2+, i.e., in the presence of 3 .times. 10-9 M and 10-8 M isoprenaline the maximal increase in heart rate was reached by 3.6 and 1.8 mM Ca2+, respectively. The dose-response curve for the positive chronotropic effect of isoprenaline was dependent on the concentration of Ca2+. At 0.45 mM Ca2+ the chronotropic effects of all concentrations were significantly lower than at 1.8 mM Ca2+. The automaticity of specialized fibers is presumably caused by a decay of potassium conductance and by an influx of Ca2+, whereas that of the sinus node is very likely due to the influx of Ca2+. Apparently both a Ca2+ influx and a decay of K efflux are involved in the generation of sinus node automaticity.