The origin of two components in contraction of guinea pig papillary muscle in the presence of noradrenaline.

In experiments on isolated guinea pig papillary muscles the effects of verapamil (5 mg/L) and caffeine (1 g/L) on the two-component contraction were investigated. The muscles were continuously superfused with normal Tyrode's solution containing 2 mg of noradrenaline/L at 20--22 degrees C. The first derivative of contractile response and transmembrane action potential were simultaneously recorded. Verapamil suppressed the amplitude of the second component and had no influence on the first component of contraction. Caffeine eliminated the first component and increased the second component of contraction. It was suggested that in the activation of two-component contraction calcium ions from two different pools take part: (1) those released from sarcoplasmic reticulum, and (2) those that crossed a cell membrane during the plateau of the action potential.