Effects of caffeine and cyclopiazonic acid on contractile proteins of adult and newborn ferret cardiac fibres

Rest-dependence of twitch amplitude and sarcoplasmic reticulum calcium content in the developing rat myocardium

Post-rest contractile response was studied in isolated ventricular muscle from rats aged 1 to 90 days. Amplitude of rapid cooling contractures (RCC) was taken as an index of the sarcoplasmic reticulum (SR) Ca2+ content. We observed that: (a) developed tension (per cross-section area) increased with age; (b) time to peak twitch force and relaxation half-time decreased from 87+/-6 to 56+/-2 ms and from 68+/-6 to 36+/-1 ms, respectively, from the neonatal period to adulthood; (c) post-rest twitch potentiation was observed at all ages, with greater relative potentiation in younger preparations, although relative potentiation of [Ca2+]i transient amplitude was similar in young and adult isolated ventricular myocytes; (d) rest did not significantly affect the amplitude of RCC in muscle or caffeine-evoked [Ca2+]i transients in myocytes at any studied age; (e) favoring Ca2+ efflux via Na+-Ca2+ exchange (NCX) during rest reversed twitch potentiation and caused a similar decrease in RCC amplitude ( approximately 40%) at all ages; (f) stimulation of Ca2+ influx via NCX during rest increased RCC amplitude ( approximately 40%) only in immature preparations. However, when this procedure was repeated after partial SR Ca2+ depletion, increase in RCC amplitude was not significantly age-dependent. We conclude that post-rest twitch potentiation is already present early after birth and does not require rest-dependent changes in SR Ca2+ content at any studied age. Our results suggest that NCX is close to equilibrium during rest in both adult and developing rat myocardium, and does not seem to mediate diastolic net Ca2+ fluxes which may affect the SR Ca2+ content.

Cardiac excitation-contraction coupling in the portal hypertensive rat

Basal contractility and responses to beta-adrenoceptor activation are compromised in hearts from rats with chronic portal vein stenosis. Here we report the effect of partial ligation of the portal vein on myocardial G protein expression, beta-adrenoceptor-G protein coupling, and excitation-contraction coupling (ECC). Contractility (dT/dt) was reduced 30-50% in right and left ventricles, but the rate of relaxation (-dT/dt) was unaffected. Isoproterenol-induced positive inotropism was diminished, but there was no difference in ED(50). The concentration-dependent increase in -dT/dt was unaffected. G(s)alpha and G(i)alpha expression, cholera toxin- and pertussis toxin-induced ADP-ribosylation, and formation of the agonist-receptor-G(s) complex were unaffected by portal vein stenosis. Of the components of ECC examined, the caffeine-sensitive sarcoplasmic reticulum Ca(2+) pool was reduced 35%, although the Ca(2+) uptake and release processes were unchanged; the apparent density of L-type Ca(2+) channels decreased 60% with no change in affinity; the dihydropyridine Ca(2+) channel agonist BAY K 8644 produced relative changes in dT/dt that were similar in both groups, suggesting normal function in the remaining Ca(2+) channels; and Na(+)/Ca(2+) exchange was reduced 50% in the portal vein stenosis group. These data suggest that the effect of portal vein stenosis on the myocardium is the result of alterations to ECC.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

An estimate of the participation of the sarcoplasmic reticulum in the intracellular Ca2+ regulation in adult and newborn ferret hearts

The aim of the present study was to estimate the participation of the sarcoplasmic reticulum in the Ca2+ regulation of the contraction of newborn ferret heart. Cyclopiazonic acid has been used to block the sarcoplasmic reticulum Ca2+ pump in adult and newborn (1 month and 5-6 day old) ferret ventricles of intact and saponin-treated preparations. Cyclopiazonic acid induced a decrease of the amplitude of the caffeine contractures generated in saponin skinned fibers. The sensitivity of the sarcoplasmic reticulum Ca2+ uptake to cyclopiazonic acid was similar in adult and newborn hearts. In intact preparations, cyclopiazonic acid (1-20 microM) induced a negative inotropic effect on the twitch with a prolongation in its kinetics. The maximal decrease in the amplitude of the twitch was larger in adult (92.4%) than in 1 month old (86.5%) and 5-6 day newborns (72.5%). Contrary to other species, where the sarcoplasmic reticulum Ca2+ pump is not functional in neonatal heart, it is proposed that ferret myocardium shows an early maturation of sarcoplasmic reticulum function.