Isoprenaline reverses the slow force responses to a length change in isolated rabbit papillary muscle

An alteration in the length of isolated cardiac muscle produces an immediate change in twitch force, then a slow further change in the same direction. We have found that the slow changes in force in rabbit papillary muscles are blocked or reversed by the ß-agonist, isoprenaline (1 M). The abolition of the slow responses by isoprenaline was not due to saturation of the myofibrils with Ca²⁺, as the blockade continued if the extracellular [Ca²⁺] was reduced in the presence of isoprenaline so that twitch force was <50% maximal. Ryanodine (1 M) did not block the slow responses, suggesting that the sarcoplasmic reticulum does not mediate the responses. These results suggest that changes of intracellular [cAMP] may mediate, or at least modulate, the slow force responses to a length change in cardiac muscle.