Role of Reduced Sarco-Endoplasmic Reticulum Ca2+-ATPase Function on Sarcoplasmic Reticulum Ca2+ Alternans in the Intact Rabbit Heart

Sarcoplasmic reticulum (SR) Ca2+ cycling is tightly regulated by ryanodine receptor (RyR) Ca2+ release and sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ uptake during each excitation–contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca2+ alternans in the intact rabbit heart, which contributes to arrhythmogenic action potential duration (APD) alternans. Recent studies have also implicated impaired SERCA function, a key feature of heart failure, in cardiac alternans and arrhythmias. However, the relationship between reduced SERCA function and SR Ca2+ alternans is not well understood. Simultaneous optical mapping of transmembrane potential (Vm) and SR Ca2+ was performed in isolated rabbit hearts (n = 10) using the voltage-sensitive dye RH237 and the low-affinity Ca2+ indicator Fluo-5N-AM. Alternans was induced by rapid ventricular pacing. SERCA was inhibited with cyclopiazonic acid (CPA; 1–10 μM). SERCA inhibition (1, 5, and 10 μM of CPA) resulted in dose-dependent slowing of SR Ca2+ reuptake, with the time constant (tau) increasing from 70.8 ± 3.5 ms at baseline to 85.5 ± 6.6, 129.9 ± 20.7, and 271.3 ± 37.6 ms, respectively (p < 0.05 vs. baseline for all doses). At fast pacing frequencies, CPA significantly increased the magnitude of SR Ca2+ and APD alternans, most strongly at 10 μM (pacing cycle length = 220 ms: SR Ca2+ alternans magnitude: 57.1 ± 4.7 vs. 13.4 ± 8.9 AU; APD alternans magnitude 3.8 ± 1.9 vs. 0.2 ± 0.19 AU; p < 0.05 10 μM of CPA vs. baseline for both). SERCA inhibition also promoted the emergence of spatially discordant alternans. Notably, at all CPA doses, alternation of SR Ca2+ release occurred prior to alternation of diastolic SR Ca2+ load as pacing frequency increased. Simultaneous optical mapping of SR Ca2+ and Vm in the intact rabbit heart revealed that SERCA inhibition exacerbates pacing-induced SR Ca2+ and APD alternans magnitude, particularly at fast pacing frequencies. Importantly, SR Ca2+ release alternans always occurred before the onset of SR Ca2+ load alternans. These findings suggest that even in settings of diminished SERCA function, relative refractoriness of RyR Ca2+ release governs the onset of intracellular Ca2+ alternans.

The influence of cholinergic drugs on EEG of the brain of the castrated rabbit males

Aim. We investigated the influence of various cholinoceptive drugs on spontaneous activity of a brain at the intact rabbit and in the conditions of deficit of gonadal hormones, and also studied of reciprocal influence of cholinergic drugs on changes of character of a range of EEG at the intact and castrated rabbits. Methods. Our researches were carried out on 32 rabbit males of breed the Chinchilla. Bipolar nichrome electrodes implanted to rabbits by stereotactic operaition into a caudate nucleus, a hippocampus and a reticular formation of a mesencephalon and also cortical monopolar chlorsilver electrodes with a diameter of 1 mm - to the optic and frontal area of cortex. Registration of EEG carried out in a shielded chamber with use of the electroencephalograph-analyzer EEGA-21/26 “Encephalan-131-03”. The averaged EEG range power was analyzed with a 60 s recording epoch. In 3 weeks after implantation of electrodes we studied the influence a number of cholinergic drugs on spontaneous EEG: M-cholinolytic metamizyl (1 mg/kg i.p.), N-cholinolytic gangleronum (5 mg/kg i.p.), inhibitor of acetylcholinesterase (AChE) galantamine (0.5 mg/kg i.p.). Registration was carried out in 1, 2 and 3 hours after an injection. Under the combined investigation of preparation galantamine was entered in 20 minutes after an injection of cholinolytics. Then, we carried out removal of a testis at rabbits — males under etherization. The spontaneous electric activity of a brain of that rabbits and influence of cholinergic drugs was registered in 30 days after operation of bilateral gonadectomy. Result. The gonadectomy of Chinchilla rabbits causes the deceleration of EEG range power twice in comparison with intact animals in a chronic experiment. The analysis of influence of cholinoceptive substances on EEG showed that the most sensitive to the change of balance between M- and N-cholinoreceptors are cerebral cortex and caudate nucleus. The intensity of the activating effect accompanying blockade of M-cholinoreceptors is higher in cortical structures, than in subcortical nuclei in all animals. The reaction of caudate nucleus to the blockade of M-cholinoreceptors is the most sensitive among subcortical nuclei. It “answers” with increase in EEG range power. The blockade of N-cholinoreceptors in this structure after insignificant potentiating is followed by decrease of EEG range power. The activity of hippocampus is least dependent on the level of androgens in organism. Conclusions. The analysis of influence of cholinoceptive substances on spontaneous activity of a brain showed that the most sensitive to change of balance between M-cholinoreceptors and H-cholinoreceptors were a cerebral cortex and a caudate nucleus. As at the intact as the castrated animals the expressiveness of the activating effect accompanying blockade of M-cholinoreceptors is higher in cortical structures than in subcortical nuclei.

Responsiveness of Coronary Arteries to Nitroglycerin under Hypoxia: The Importance of the Endothelium

Background/Aims: Nitroglycerin is widely used as a coronary vasodilator in the treatment of ischemic heart diseases. This study investigated the influence of hypoxia on nitroglycerin-induced relaxation in endothelium-intact and -denuded rabbit, monkey, and porcine coronary arteries. Methods: Helically cut strips of coronary arteries were suspended in organ chambers, and isometric tension was recorded. Results: Nitroglycerin concentration dependently relaxed endothelium-intact rabbit coronary arteries, which were not different under normoxic and hypoxic conditions. On the other hand, nitroglycerin-induced relaxation of endothelium-denuded arteries was significantly attenuated by hypoxia. Similarly, the relaxant response of endothelium-intact monkey coronary arteries to nitroglycerin was not affected by hypoxia, whereas that of endothelium-denuded arteries was impaired. As is the case with rabbit and monkey coronary arteries, exposure to hypoxia resulted in impaired relaxation by nitroglycerin in endothelium-denuded but not endothelium-intact porcine coronary arteries. Conclusion: These findings suggest that coronary endothelium plays a pivotal role in preventing the hypoxia-induced impairment of nitroglycerin responsiveness, regardless of the animal species.