Organ-cultured chick embryonic hearts of various ages. II. Ultrastructure

Effects of low sodium ouabain and amiloride on chick myocardial contraction

1. Effects of Ca2+, low Na+, ouabain and amiloride on contractile force were examined in myocardial preparations from embryonic and hatched chicks. Measurement of contractile force was performed in an organ bath with whole hearts for the young embryo (5-6 days old) and with isolated strips from the right ventricles for the old embryos (16-18-day-old), hatched chicks (within 24 hr after hatching) and 1-week-old chicks. 2. The maximum increase in contractile force above the basal value at each age was determined by increasing extracellular Ca2+, which was smaller in young embryonic hearts than in older ones. 3. Decreasing the Na+ concentration to half of the normal value increased the contractile force in all ages examined: the increase in contractile force expressed as a percentage of the maximum increase at each age was significantly larger in young embryonic hearts than in older ones. 4. Ouabain produced increases in contractile forces at all ages examined; the effective concentration was lower in young embryonic hearts than in older hearts. 5. Amiloride prolonged the relaxation phase of contraction at all ages examined; the degree of prolongation was larger in young embryonic hearts than in older ones. 6. The present results suggest that, in the chick myocardium, the relative role of Na(+)-Ca2+ exchange in relaxation decreases during embryonic development, which may reflect the concurrent increase in sarcoplasmic reticulum function [Tanaka, Takagi and Shigenobu (1993) J. Dev. Physiol. 19, 235-240.

Perfluorocarbon-based medium for culture of the early chick embryo heart

In an attempt to prolong the survival of the explanted early chick embryo heart, hearts at stages 10 to 28 were cultured in supplemented Dulbecco's modified Eagle's medium with or without the perfluorocarbon, perfluorotributylamine. The perfluorocarbon was added to the standard culture medium in a 50:50 (vol/vol) mixture. Explants were evaluated daily and were harvested for light microscopy after 2 to 10 d in culture. The tubular shape of the explants was generally maintained for 2 d in culture, after which the hearts became dilated or spherical. Beating was noted in some of the explants on Day 2 in culture but not thereafter. Microscopic evaluation showed patchy areas of necrosis in all explants by Day 3, although large areas of viable epithelioid cells were documented as long as 7 d after explanation. Stage 16 to 18 hearts cultured in the presence of perfluorocarbon were more likely to maintain tubular architecture on microscopy than hearts cultured in standard medium. Hearts cultured from later stages showed no improvement in appearance with the presence of perfluorocarbon and there was a suggestion of increased necrosis in later-stage explants cultured with perfluorocarbon for 4 d. Further modification of the culture system will be required to prolong explant survival and development beyond 2 d.

Effects of calcium channel blockers on ouabain-induced oscillatory afterpotentials in organ-cultured young embryonic chick hearts

Ouabain induces oscillatory afterpotentials (OAPs) in organ-cultured young (3 day old) embryonic chick hearts. Since increased [Ca]i resulting from an inhibition of the Na pump by ouabain triggers oscillatory movements of Ca2+ (i.e. OAPs) intracellularly, Ca2+ influx through the cell membrane, which tends to increase [Ca]i, may be important in developing the OAPs. Therefore, in the present experiments, effects of calcium channel blockers on ouabain-induced OAPs in organ-cultured 3 day old embryonic chick hearts were examined. Automaticity was suppressed by elevating [K]o to 6 mM. To induce the OAPs, the preparations were stimulated (0.5 Hz) in the presence of ouabain (2.5-6.3 microM). The calcium channel blockers (10 microM) depressed the OAPs in the following order of potency: bepridil greater than verapamil greater than nifedipine greater than diltiazem. This order of potency of the calcium channel blockers in depressing the OAPs was the same as that for drug penetration into the cells, but different from that for depressing slow action potentials: nifedipine greater than diltiazem greater than verapamil greater than bepridil (our previous findings). These results suggest that an intracellular site of action of the calcium channel blockers is important for depression of the OAPs, and suppression of the slow inward Ca2+ current cannot be the sole mechanism for suppression of the OAPs by these drugs.

Messenger RNA induction of fast sodium ion channels in cultured cardiac myoblasts

Incubation with adult heart messenger RNA caused the appearance of fast sodium ion channels in young myocardial cells whose development has been arrested in vitro. The induction was blocked by cycloheximide, indicating dependence on protein synthesis. Thus, cardiac myoblasts can be made to differentiate in vitro, and membrane properties can be altered by exogenous RNA.