Effect of calcium binding protein modulators on myofibrillar MgATPase activity and cGMP‐inhibitable phosphodiesterase activity from human cardiac muscle

Abstract

AbstractCardiac muscle force development and shortening result from the interaction between actin and myosin within the myofibrillar contractile unit. This interaction is dependent upon intracellular Ca2+ and is controlled by the troponin‐tropomyosin regulatory proteins situated along the length of the actin thin filament. Enhancing the Ca2+ sensitivity of cardiac contractile protein interactions has been proposed as a novel mechanism for some positive inotropic agents. These myofibrillar Ca2+ sensitizers have variable effects on myofibrillar MgATPase activity among nonhuman animal species, and different effects between myofibrils from failed nonhuman hearts are evident. In this study, we have evaluated several myofibrillar calcium sensitizers, at maximal testable concentrations, for their effects on the calcium‐activated MgATPase of myofibrils prepared from normal human hearts and from hearts of patients with congestive heart failure. Bepridil (100 μM) increased MgATPase at pCA 9.0 (basal activity) and pCa 6.5 (an intermediate concentration of Ca2+ which stimulated activity 42–56% of maximal), with no effect at pCa 5.0 (maximal Ca2+ activity) in both normal and failing hearts. APP 201–533 (300 μM) increased MgATPase at pCa 6.5 in all hearts, but increased MgATPase at pCa 5.0 only in normal hearts. Pimobendan (100 μM) increased MgATPase at pCa 6.5 only in normal hearts and increased activity at pCa 5.0 only in failing hearts. In contrast, trifluoperazine (100 μM) reduced MgATPase at pCa 5.0 and pCa 9.0 in all hearts and at pCa 6.5 in failing hearts. The stimulating effects of bepridil, APP 201–533, and pimobendan (approximately 7–17%) were less than those observed in studies with nonhuman cardiac myofibrils (typically 30–40%). These data suggest that some myofibrillar Ca2+ sensitizers can have either modest stimulatory or inhibitory activity on human cardiac myofibrillar MgATPase activity. While these (Ca+‐sensitizing) effects on MgATPase activity are statistically significant, the biological relevance with respect to enhancing cardiac contractility is equivocal. Furthermore, both pimobendan and APP 201–533 inhibited cAMP‐phosphodiesterase III purified from failing human hearts with IC50 values of 0.35 and 2.0 μM, respectively. Since other inotropic mechanisms, such as cAMP‐phosphodiesterase inhibition, are also evident at much lower concentrations with some of these agents, the relevance of myofibrillar Ca2+ sensitization as the predominant inotropic mechanism of action of these compounds is questionable. © 1993 Wiley‐Liss, Inc.