At least three functional isoforms of the cardiac Na(+)-Ca2+ exchange exist

Decreased inotropic response to beta-adrenergic stimulation and normal sarcoplasmic reticulum calcium stores in the spontaneously hypertensive rat heart

Cardiac hypertrophy in the spontaneously hypertensive rat has been shown to be accompanied by a diminished inotropic response to beta-adrenergic stimulation. This diminished response has been attributed to abnormalities in various components of the beta-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 diminished response to beta-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 probe 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 beta-adrenergic stimulation. We observed a significant decrease in the inotropic response to beta-adrenergic stimulation in papillary 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 beta-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 mitochondrial and at the level of the myofilaments by electron probe microanalysis. These results indicate that intracellular Ca2+ overload does 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.

Cultured chick-embryo heart cells respond differently to ouabain as measured by the increase in their intracellular Na+ concentration

Using digital imaging microscopy with the sodium-sensitive fluorescent indicator sodium-binding benzofuran isophtalate (SBFI), we examined the cytosolic free sodium ion concentration ([Na+]i) in single chick-embryo heart cells. The distribution of the [Na+]i was homogeneous within one cell, but we found a wide cell to cell variation in the range of 3 to 18 mM [Na+]i. In contrast to former experiments showing a heterogeneity of chick-embryo heart cells with respect to their [Ca2+]i (Ahlemeyer et al. (1992) Eur. J. Biochem. 205, 269-275), we could not distinguish cell populations with different [Na+]i. We found a lognormal distribution of the resting [Na+]i with a median value of 8.8 mM with a standard deviation of 4.5 mM (n = 90). After the addition of varying concentrations of ouabain, we found a biphasic dose-response curve as measured by the increase in [Na+]i. Ouabain showed its half-maximal effect on the [Na+]i between 10(-9) M and 10(-8) M and at 4.3.10(-6) M under steady-state conditions. The finding of a heterogeneity of chick-embryo heart cells with respect to their ouabain-induced increase in [Na+]i is consistent with our previous observations of cells differing in their [Ca2+]i and in the sensitivity of their sodium pumps to cardiac glycosides.