Diminished Inotropic Response to Amrinone in Ventricular Myocytes from Myopathic Hamsters Is Linked to Depression of High-Gain Ca²⁺-Induced Ca²⁺Release

Abstract

This study investigates whether amrinone (100 -1000 M), a phosphodiesterase-III inhibitor, can alleviate depression of contractions in ventricular myocytes from prefailure cardiomyopathic (CM) hamsters (80 -100 days). Cell shortening and ion currents were measured in voltage-clamped cells at 37°C. Normal myocytes exhibited low-gain Ca 2ϩ -induced Ca 2ϩ release (CICR) initiated by test steps from Ϫ40 mV and high-gain CICR initiated from more negative potentials. In normal myocytes, amrinone selectively increased contractions initiated by highgain CICR (fractional shortening increased from 3.6 Ϯ 0.5% to 5.3 Ϯ 0.6%, 300 M amrinone) but had no effect on low-gain CICR. Amrinone decreased L-type Ca 2ϩ current (I Ca-L ; Ϫ5.5 Ϯ 0.8 to Ϫ3.7 Ϯ 0.5 picoAmp/picoFarad, 300 M amrinone). In contrast, in CM myocytes, high-gain CICR was virtually absent, and amrinone had no inotropic effect. Amrinone inhibited I Ca-L less in CM than normal myocytes. Sarcoplasmic reticulum (SR) Ca 2ϩ stores, assessed by caffeine, were significantly increased by amrinone in normal but not CM myocytes. Thus, the positive inotropic effect of amrinone in normal hamster myocytes was mediated by selective enhancement of high-gain CICR. This effect was not mediated by stimulation of I Ca-L because I Ca-L is inhibited by this drug in hamster. High-gain CICR, which is depressed in CM myocytes, cannot be restored by amrinone. However, minimal stimulation of adenylyl cyclase with forskolin restored the positive inotropic effect of amrinone in CM cells. This positive inotropic effect of amrinone may reflect increased SR Ca 2ϩ stores because increased stores accompanied the positive inotropic effect in normal myocytes but were absent in CM myocytes.

Contractile responses to selective phosphodiesterase inhibitors following chronic β-adrenoreceptor activation

“…Inotropic responses to selective PDE inhibitors are reduced in heart failure [4,10,33,36]. The present study is the first to suggest that these changes can be attributed directly to sympathetic overactivation and abnormalities in signal transduction, rather than to diffuse myocardial damage and contractile dysfunction.…”

“…However, most studies performed to date have described PDE-inhibitor-mediated effects in heart failure where cardiac changes may occur that are not necessarily consequences of a downregulation of the β-AR-cAMP system. Indeed, intrinsic alterations in Ca 2+ cycling [15,36] and profound myocardial damage [24], as well as myocyte dysfunction [1,9], could potentially contribute to a reduction in PDE-inhibitor-induced responsiveness. Therefore, whether reduced PDE-inhibitor-induced contractile responses are attributed to sympathetic activation and subsequent alterations in signal transduction systems or to diffuse myocardial damage and dysfunction is unclear.…”

Myocardial Phosphodiesterases and Regulation of Cardiac Contractility in Health and Cardiac Disease

“…Selective PDE3 inhibitors-induced I Ca2+ stimulation was shown to be significantly larger in human as compared to rabbit atrial myocytes [95]. In contrast, amrinone, a selective PDE3 inhibitor, reduced rather than increased I Ca2+ amplitude in isolated hamster ventricular myocytes [119]. Collectively, these findings suggest species-related differences in PDE inhibitors effect on I Ca2+ amplitude under resting conditions.…”

“…Indeed, in isolated hamster ventricular myocytes, amrinone, a selective PDE3 inhibitor, stimulates sarcoplasmic reticulum voltage-sensitive Ca 2+ release mechanism, an effect occurring independently of changes in I Ca2+ amplitude [112,119]. Furthermore, PDE3 inhibitors are capable of to increase Ca 2+ stores in sarcoplasmic reticulum [119,124], and directly promote sarcoplasmic reticulum Ca 2+ release via ryanodine-sensitive mechanism [98,104,125,126]. There is some evidence to indicate that PDE3 inhibitors could promote Na + -Ca 2+ exchange [105] and increase myocardial membrane Ca 2+ -ATPase activity [127].…”

Pivotal effects of phosphodiesterase inhibitors on myocyte contractility and viability in normal and ischemic hearts

“…The positive inotropic effects of milrinone in guinea pig [82] and dog [83] can be attenuated by a calcium channel blocker. Interestingly, the effect of PDE3 inhibition may differ from species to species as amrinone reduced ICa in normal hamster ventricular myocytes and produced no inotropic effects in cardiomyopathic hamsters [84] . There is also evidence that PDE3 inhibitors cause positive inotropy independently of increases in ICa and are likely due to increases in cAMP and PKA function in other localized areas resulting in altered calcium handling by SR. For instance, milrinone but not enoximone stimulates Ca release from SR by RyR2 in sheep [85] .…”