Cardiac hypertrophy in the spontaneously hypertensive rat has been shown to be accompanied by a diminished inotropic response to β-adrenergic stimulation. This diminished response has been attributed to abnormalities in various components of the β-adrenergic signaling system. There is also evidence that regulation of intracellular Ca2+ cycling may be altered in the hypertrophied heart of the spontaneously hypertensive rat. We proposed that the dominished reponse to β-adrenergic stimulation may reflect abnormalities in Ca2+ cycling, specifically alterations in the ability of the sarcoplasmic reticulum to effectively release and resequester ca2+. We have used the unique combination of functional measurements on isolated, isometrically contracting papillary muscles from hearts of 26-week-old spontaneously hypertensive rats and their Wistar-Kyoto controls, together with electron proble microanalysis measurements of sarcoplasmic reticulum Ca2+ content in the same muscles after rapid freezing, to determine the availability of Ca2+ for activation of contraction, following β-adrenergic stimulation. We observed a significant decrease in the inotropic response to β-adrenergic stimulation in paillary muscles from the spontaneously hypertensive rats, as compared with Wistar-Kyoto controls, however in these same muscles, frozen during relaxation. there was no evidence of an accompanying decrease in the size of the sarcoplasmic reticulum Ca2+ store. In an additional group of muscles which were frozen during contraction, the amount of Ca2+ remaining in the sarcoplasmic reticulum after stimulated release was also not different in the two strains. These results indicate that the decreased inotropic response to β-adrenergic stimulation in hypertrophied hearts of the spontaneously hypertensive rat is unlikely to be due to decreased availability of Ca2+ for activation of contraction. Additionally, to determine whether there is intracellular Ca2+ overload in the cardiac muscle cells of hearts of spontaneously hypertensive rats, we measured the amount of Ca2+ in mitochondria and at the level of the myofilaments by electron probe microanalysis. These results indicate that intracellular Ca2+ overload doest not accompany cardiac hypertrophy in the spontaneously hypertensive rat. This study therefore shows no correlation between altered intracellular Ca2+ cycling and the decreased inotropic response to isoproterenol in the spontaneously hypertensive rat at 26 weeks of age.
- Cardiac hypertrophy
- Sarcoplasmic reticulum
- Spontaneously hypertensive rat
- β-adrenergic stimulation
ASJC Scopus subject areas
- Molecular Biology
- Cardiology and Cardiovascular Medicine