The action of forskolin on muscle cells is modified by hormones, calcium ions and calcium antagonists.

Forskolin, a drug isolated from the Indian plant Coleus forskohlii, exerts its actions on cells by directly activating the catalytic subunit of adenylatecyclases. The primary effect on heart muscles is the positive inotropic one, at higher forskolin concentrations, an acceleration of the pacemaker activity can be observed. External Ca2+ is required for this augmentation of contraction, verapamil, prenylamine and tetrodotoxin depress these effects. The action of forskolin and digitalis glycosides are to a certain degree additive. Incubation of rat heart slices with forskolin depresses the membrane bound Na+K+-ATPase activity. In smooth muscle cells, forskolin lowers the contractions, evoked by K+, norepinephrine and by angiotensin II. The results are discussed in respect to a forskolin induced enlargement of the Ca2+-uptake of heart muscle cells and a lowering of the Ca2+-sensitivity of the contractile system of smooth muscle cells.