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

Abstract

Cardiac 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 i s 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 FM) increased MgATPase at pCA 9.0 (basal activity) and pCa 6.5 (an intermediate concentration of Ca2+ which stimulated activity42-56% of maximal), with no effect at pCa5.0 (maximal Ca2+ activity) in both normal and failing hearts. APP 201-533 (300 pM) increased MgATPase at pCa 6.5 in all hearts, but increased MgATPase at pCa 5.0 only in normal hearts. Pimobendan (100 pM) 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 KM) 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 3040%). 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 i s equivocal. Furthermore, both pimobendan and APP 201-533 inhibited CAMP-phosphodiesterase Ill purified from failing human hearts with I C, , values of 0.35 and 2.0 pM, 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. o 1993 WiIey-Liss, Inc.

A review of the pharmacology and clinical uses of pimobendan.

“…Furthermore, patients with reduced ventricular function were more sensitive to these effects when compared with patients with normal ventricular function . However, other data suggested that pimobendan's inotropic effect on the failing human myocardium was predominantly due to inhibition of PDE3 . Studies have also evaluated the mechanism of action of pimobendan's active metabolite.…”

“…17 However, other data suggested that pimobendan's inotropic effect on the failing human myocardium was predominantly due to inhibition of PDE3. 68 Studies have also evaluated the mechanism of action of pimobendan's active metabolite. The PDE3 inhibitory action of UD-CG 212 Cl was significantly more potent than that of pimobendan.…”

Calcium‐Sensitizing Drugs: Positive Inotropy by Enhanced Sensitivity of the Contractile Apparatus to Calcium

“…Most drugs affect the enzyme indirectly, via enhanced calcium binding to the myofibrils, most probably to troponin-C. As enhanced calcium binding can only be interpreted in terms of more troponin-C molecules binding calcium to their regulatory site, it is obvious that more myosin heads will be recruited to generate crossbridges, resulting in an equivalent increase in myosin ATPase activity. This scenario has been reported for most drugs described above (29,58,70,82,103,124), with the exception of levosimendan (38).…”

“…In fact, compounds developed in the early 1980s were predominantly phosphodiesterase-111 inhibitors, and showed only modest calciumsensitizing activity at much higher doses ( Table 1). Although most investigators interpreted their results generally as calcium sensitization at least being involved in the particular drug's positive inotropic effect, the relevance of sensitization at much higher concentrations has been questioned (82). More recently developed compounds have been reported to have predominantly calcium-sensitizing activity.…”

“…Pagani et al (82) evaluated the effect of three drugs with calcium-sensitizing properties on myofibrillar ATPase and phosphodiesterase activity from both normal and congestive heart failure tissues. Pimobendan, bepridil, and APP201-533 had a significant but relatively modest effect in sensitizing myofibrils from non-failing and failing human heart to calcium (7-17%), when compared with non-human myofibrils (typically 3040%).…”