Activation of purified cardiac ryanodine receptors by dihydropyridine agonists

Prior observations have raised the possibility that dihydropyridine (DHP) a gonists directly affect the sarcoplasmic reticulum (SR) cardiac Ca2+ releas e channel [i.e., ryanodine receptor (RyR)]. In single-channel recordings of purified canine cardiac RyR, both DHP agonists (-)-BAY K 8644 and (1)-SDZ2 02-791 increased the open probability of the RyR when added to the cytoplas mic face of the channel. Importantly, the DHP antagonists nifedipine and (- ) SDZ202-791 had no competitive blocking effects either alone or after chan nel activation with agonist. Thus there is a stereospecific effect of SDZ20 2-791, such that the agonist activates the channel, whereas the antagonist has little effect on channel activity. Further experiments showed that DHP agonists changed RyR activation by suppressing Ca2+ induced inactivation of the channel. We concluded that DHP agonists can also influence RyR single- channel activity directly at a unique allosteric site located on the cytopl asmic face of the channel. Similar results were obtained in human purified cardiac RyR. An implication of these data is that RyR activation by DHP ago nists is likely to cause a loss of Ca2+ from the SR and to contribute to th e negative inotropic effects of these agents reported by other investigator s. Our results support this notion that the negative inotropic effects of D HP agonists result in part from direct alteration in the activity of RyRs.