Effects of endocardial endothelium in myocardial mechanics of hypertrophied myocardium of rats

AIM

To investigate the effects of endocardial endothelium (EE) in myocardial mechanics of hypertrophied myocardium of rats.

METHODS

Hypertrophied myocardium has been developed through long-term renovascular hypertension in rats. Selective stripping of EE was achieved chemically and the effects of EE on the activation and relaxation of hypertrophied myocardium were investigated and compared to those in the presence and absence of intact EE.

RESULTS

Resting tension (RT) and peak developed tension (DT) of isometric contraction of the left ventricular papillary muscles of renovascular hypertensive rats (RHR) remained similar, but maximal rate of tension development and fall (+dT/dtmax and -dT/dtmax) were lowered, time to +dT/dtmax (TPP) and time to -dT/dtmax (TPN) were increased, and time to peak developed tension (TPT) and half relaxation time (RT1/2) were prolonged, vs sham-operated rats (Sham). After EE denudation, TPT, and RT1/2 were significantly abbreviated in RHR. The -dT/dtmax was significantly augmented while the +dT/dtmax was unaltered, resulting in a prominent decrease in the ratio of +dT/dtmax to -dT/dtmax.

CONCLUSION

EE predominantly influences the relaxation in isolated myocardium and early diastolic filling events in hearts, and is involved in the cardiac compensatory mechanism in hypertrophied myocardium.

Endocardial endothelium selectively modifies relaxation in rat papillary muscle

The selective removal of endocardial endothelium of rat left ventricular papillary muscles by 1-second immersion in 0.5% Triton X-100 showed little influence on resting tension and only a small decrease in peak isometric tension (8.3 +/- 1.4 vs 9.6 +/- 2.4 mN/mm2 at Lmax, p > 0.05) with no reduction in maximal rate of tension development (+dT/dtmax; 136 +/- 21 vs 137 +/- 18 mN/mm2/s, p > 0.05). In contrast, there was a marked increase in maximal rate of tension decline (-dT/dtmax) from 71 +/- 14 to 92 +/- 15 mN/mm2/s (p < 0.05), so that the ratio between +dT/dtmax and -dT/dtmax fell from 1.98 +/- 0.27 to 1.51 +/- 0.13 (p < 0.01). Removal of endocardial endothelium led to a significant shortening of isometric twitch contractions. Time to peak tension was abbreviated from 111 +/- 20 to 84 +/- 8 ms (p < 0.05) and the half relaxation time from 92 +/- 9 to 68 +/- 8 ms (p < 0.01). Time to +dT/dtmax was also shortened from 31 +/- 6 to 44 +/- 9 ms (p < 0.05) and time to -dT/dtmax from 90 +/- 12 to 62 +/- 10 ms (p < 0.01). These effects were not influenced by alterations in stimulation frequency or muscle length. The early onset of relaxation and abbreviated duration of relaxation together with an increased rate of decline in tension led to a shorter total twitch which may explain the slightly lower peak tension once the endocardial endothelium was removed. Our findings confirm that endocardial endothelium modulates myocardial contraction, with a predominant influence on relaxation.

Endocardial endothelium modulates cardiac responses to histamine and impromidine in isolated working right ventricle of guinea pigs

In isolated working right ventricle of guinea pigs, the hypothesis that endocardial endothelium (EE) might be involved in the modulation of cardiac responses to histamine receptor agonists was tested. The functional EE was denuded by switching Krebs perfusion solution to the solution containing saponin (30 micrograms.ml-1) for 2 min at a rate of 16 ml.min-1, followed by thorough washing with Krebs solution. The cardiac responses to histamine receptor agonists were compared in the presence and absence of EE. Bolus injection of histamine 0.5 mg into right ventricle elevated the right ventricular pressure (RVP), +dP/dtmax, and -dP/dtmax by 11%, 17%, and 35%, respectively, in the presence of intact EE; whereas by 30%, 43%, and 92%, respectively, after chemically selective denudation of EE with saponin. Similarly, impromidine (a H2 receptor agonist) 1, 3, 9, 27, and 54 micrograms obviously potentiated the RVP and +/- dP/dtmax in a concentration-dependent manner in preparations either with or without EE. The effects, however, could be greatly enhanced in the absence of EE. Pulmonary outflow was declined at 27 micrograms impromidine in EE-removed group. The results suggested that the augmentation of cardiac responses produced by histamine receptor agonists in EE-denuded preparations might be due to blockade of release of endothelium-derived relaxing factor, resulting in a reduced abbreviating effect on the myocardial contraction.

Endocardium modulates myocardial contractile performance in isolated guinea pig papillary muscles

Selective removal of endocardium by 1-s immersion of the muscle into 0.5% Triton X-100 resulted in a significant reduction of PT (peak isometric twitch tension) at 1.25 mmol.L-1 [Ca2+]o over the stimulation frequency from 0.2 to 2.0 Hz (3.2 +/- 0.2 vs 4.4 +/- 0.4 mN/mm2 at 1 Hz, P < 0.01), while +dT/dtmax was unaltered. Tension-[Ca2+]o relation was shifted in accordance to [Ca2+]o, but with no significant change on PT at high [Ca2+]o compared with endocardium-intact muscles. TPT (time to peak isometric tension) and RT1/2 (half isometric relaxation time) were typically shortened at all [Ca2+]o or various stimulation frequencies (TPT: 203 +/- 18 vs 265 +/- 37 ms; RT1/2: 77 +/- 10 vs 108 +/- 26 ms, at 1.25 mmol.L-1 and 1 Hz, P < 0.01). Stimulation duration-threshold curve was slightly shifted to the left, yet no change in ERP (effective refractory period) was found. The data demonstrated that endocardium was an important modulator of myocardial contractile performance.

Anti-lipid peroxidation and protection of ginsenosides against cerebral ischemia-reperfusion injuries in rats

The correlation between protective effect of ginsenosides Rb + R0 and brain endogenously-derived prostacyclin synthesis, thromboxane A2 formation and lipid peroxidation were estimated in rats. Ginsenosides Rb + R0 100 mg/kg iv 30 min before 4-vessel occlusion elevated 6-keto-PGF1 alpha level, declined thromboxane B2 and brain edema formation, reduced the rise of lipid peroxides and suppressed the reduction in both creatine phosphokinase (CK) and superoxide dismutase (SOD) activities in brain tissue after 40-min ischemia followed by 1-h reperfusion. Furthermore, these improvements were partially abolished by pretreating with iv indomethacin. It is concluded that ginsenosides possess protective effect on cerebral ischemia-reperfusion injury of rats and ginsenosides Rb + R0 are the active principles. The underlying mechanism of protection is ascribed partially or mainly to the facilitated synthesis and release of prostacyclin, reduced formation of thromboxane A2 and inhibited generation of free radicals and subsequent lipid peroxidation.