Action of ionophore A23187 on the strength of contraction and transmembrane action potential of guinea pig papillary muscle

The complex structure of mammalian heart contraction

Guinea pig papillary muscle contraction at 25 degrees C and 2 mg/liter norepinephrine (NE) consists of two components (C1, C2). Their parameters were determined in an analysis of contraction after abrupt load shifts. The C1 amplitude increased by 81 +/- 22% (N = 9) when stimulation frequency was raised from 0.1 to 1.0 Hz; its time to peak was constant at 169 +/- 6 msec (N = 12). C1 was abolished by 2 mM caffeine. The amplitude of C2 was constant between 0.1 and 0.5 Hz but decreased to 19 +/- 10% (N = 4) at 1.0 Hz; its time to peak correlated with the duration of action potential (DAP) (r = 0.98) and decreased from 518 +/- 12 msec at 0.1 Hz to 314 +/- 20 msec at 1.0 Hz (N = 12). C2 was eliminated by 5 mg/liter verapamil. Increase of DAP by a current sent through the muscle in the single sucrose gap technique gave rise to a C3 with time to peak equal to the new DAP. Mammalian heart muscle contraction in the presence of NE is supposed to be the sum of components elicited by Ca release (and after-release) from sarcoplasmic reticulum and a tonic component evoked by Ca release from sarcolemmal binding sites.