| Cardiac glycosides(CGs) had been used for the heart failure for more than 200 years. But the mechanism of its positive inotropic effect is still controversial. It is generally believed that CGs produce positive inotropic actions in failure heart via the inhibition of Na+,K+-ATPase activities which results in the elevation of bulk intracellular Na+ concentration([Na+]), and subsequent increase of intracellular Ca2+ concentration ([Ca2+]i) via Na+-Ca2+ exchange, thus increasing the uptake and subsequent release of Ca2+ by the sarcoplasmic reticulum (SR). However, the therapeutic concentrations (1~10 nmol/L, low concentrations) of CGs to produce the positive inotropic effects in patients with congestive heart failure are much lower than those shown to inhibit the Na+,K+-ATPase in vitro (1~100 μmol/L). In deed a stimulatary effect or no effect of Str on Na+,K+-ATPase at low concentrations has been found. Thus, the Na+,K+-ATPase inhibition theory seems to be inadequate to explain the inotropic action of CGs at the therapeutic levels. Furthermore, the reports about the effects of glycosides at low concentrations on cardiac performance were contradictory, which showed a positive inotropic effect in some cases, a negative inotropic, or no effect in other experiments. Sothe relationship between the inotropic actions of Str and its effects on Na+, K+-ATPase activities at low concentrations needs to be confirmed. The elevation of [Ca2+]i is a key factor in the excitation-contraction coupling of cardiomyocytes and is also thought to be the mechanism of CGs to produce positive inotropic effect. Generally, the possible routes of [Ca2+]i elevation include extracellular Ca2+ influx (via ①"slip-mode Ca2+ conductance'' of TTX-sensitive Na+ channel ② L-type or receptor-operated Ca2+ channel ③ Na+/Ca2+ exchanger) and intracellular Ca2+ release from SR through ryanodine receptor. In the present study, we first explored the relationship between the cardiotonic effects of Str and its effects on Na+,K+-ATPase both in normal and failure isolated guinea-pig heart. Then we assessed the possible mechanism of [Ca2+]i elevation with confocal microscopy and whole-cell patch-clamp technique. The experimental projects and results are as follows.1．The relationship between the cardiotonic effects and the cardiac sarcolemmal Na+, K+-ATPase activities by strophanthidin in isolated guinea-pig heartAIM: To evaluate the effects of strophanthidin (Str) 0.1, 1, 10, 100 nmol/L and 1,10,100μmol/L on cardiac contractile function and sarcolemmal Na+, K+-ATPase activities in isolated guinea-pig hearts. METHODS: Isolated guinea-pig hearts were perfused through aorta in a Langendorff mode. Heart rate (HR), left ventricular pressure (LVP), and firstderivatives (±dp/dtmax) of LVP were recorded by eight-channel physiological instrument. Cardiac sarcolemmal Na+, K+-ATPase activities were determined with colorimetry. RESULTS: ⑴ Effects of Str on cardiac contractility: In the control group, perfused guinea-pig hearts were continuously observed for 50 min and showed no changes in HR, LVP and ±dp/dtmax. Str 0.1 nmol/L did not affect the cardiac contractile function (P > 0.05); Str 1 nmol/L increased +dp/dtmax (P < 0.05), but had no effects on other parameters; Str 10~ 100 nmol/L increased both LVP and +dp/dtmax (P < 0.05 or P < 0.01). The peaks of LVP and +dp/dtmax were reached within 10~15 min and then lasted for whole observation. There was no obvious change in HR and -dp/dtmax in all of Str groups above. Str 1, 10 and 100 μmol/L first caused an increase (P < 0.01) (A peak effect was reached within 10 min) and subsequent a decrease in LVP and ±dp/dtmax accompanied by the irregular contraction ( the incidence of irregular contraction were 57 %, 75 % and 100% respectively). A concentration-dependent decrease in HR was observed ( P < 0.05 or P < 0.01 respectively). ⑵ Effects of Str on cardiac sarcolemmal Na+,K+-ATPase activities: The electron microscopic examination showed that the membrane fraction consis...
|
PDF Full Text Request