Coronary smooth muscle reactivity to repeated agonist administration in a long-time experiment in vitro

The reactivity of isolated extramural coronary arteries of the pig to repeated agonist administration (acetylcholine, prostaglandin F2 alpha and K+-contraction) was investigated in a long-time experiment (20 h, more than 60 reactions). The acetylcholine-evoked contractions show an amplitude increase (phasic component) during the first 10 responses followed by approximately 20 constant reactions and a final amplitude decrease with alternative changes in amplitude. The tonic component of the acetylcholine responses decreases uniformly. The amplitude of K+-contraction increases during the first 10 reactions, and remains nearly constant in the further experiment. Prostaglandin F2 alpha produces a maximum contraction already in its first response, but the amplitudes decrease considerably after 6-8 reactions. The time course of the corresponding contraction upstroke velocity is quite similar compared to the amplitude behaviour. Differences in the smooth muscle reactivity between the used agonists are explained by different activatory mechanisms. Some hints concerning an optimum experimental procedure are given.