Action potentials (APs) and the force of isometric contractions of the right ventricular papillary muscles were measured in adult and newborn guinea-pigs. The measurements were carried out in the steady state with the rate of stimulation of 0.1, 1, and 2 Hz, and further some measurements were done in which Sr2+ was substituted for Ca2- The duration of APs of the newborn animals without pharmacological treatment was significantly shorter in comparison with that of the adults at all the used stimulation frequencies. An analogous sensitivity was found in the contractile force to increased stimulation frequency and when the steady state stimulation was discontinued by the insertion of interpolated extrasystoles in papillary muscles of adult or newborn animals. The biphasic contractions of papillary muscles were evoked in both groups of animals by the incomplete substitution of Sr2+ by Ca2+ in the presence of isoprenaline. The early component of the biphasic contractions had a faster course as compared to the late component and disappeared in the presence of caffeine in both groups of animals. Our results suggest that the heart cells of newborn guinea-pigs probably possess the sarcoplasmic reticulum (SR), whose function does not differ in quality from that of the adult guinea-pigs. The postnatal prolongation of APs is therefore not probably the result of postnatal development changes of the functions of SR, but could be related to changes in the relations between the surface and volume of the heart cell during its growth.
The effect of low (1 mg/animal/day), medium (10 mg/animal/day) and high (100 mg/animal/day) intake of ascorbic acid on tissue lipid peroxidation (LPO) and the physical state of erythrocyte membranes was investigated in female guinea-pigs fed a vitamin E low diet. Animals were killed after 9-11 weeks and the blood, liver, lungs, kidneys and adrenals were analysed. The LPO was estimated by the determination of malondialdehyde with HPLC. The physical state of erythrocyte membranes was determined spectrofluorometrically and expressed as fluorescence polarization of membrane lipid specific probe 1,6-diphenyl-1,3,5-hexatriene. The LPO concentrations in the liver and adrenals of the group on a low vitamin C intake were significantly increased. A significant non-linear negative correlation between C vitamin levels and LPO concentrations was found in these tissues. The fluidity of erythrocyte membranes as a measure of their structural state was significantly lower in the group with a low intake of C vitamin. It is probable that the water-soluble antioxidants, such as vitamin C, act in the plasma as primary defense against oxidative stress if the radicals are formed initially in the aqueous phase of whole blood.