Developmental changes in excitation-contraction mechanisms of the mouse ventricular myocardium as revealed by functional and confocal imaging analyses.

Developmental changes in excitation-contraction mechanisms were examined in the ventricular myocardium from fetal, neonatal, and 1-, 2-, and 4-week-old mice. In isolated tissue, the negative inotropic effect of nifedipine decreased, while that of ryanodine increased with age. Action potential duration decreased with age, especially during the late fetal period. In ventricular cardiomyocytes, fluorescence imaging revealed that the sarcoplasmic reticulum increases progressively during pre- and postnatal development. t-Tubules were absent in the fetus and neonate, were observed only in the subsarcolemmal region at 1 week after birth, and were present throughout the cytoplasm at 2 and 4 weeks after birth. The amplitude of Ca(2+) transients, as well as its ryanodine-sensitive component, increased with age. In the neonate and 1-week-old mice, Ca(2+) at the cell center showed slower rise than the subsarcolemmal region, but in 2- and 4-week-old mice, Ca(2+) increased simultaneously across the entire width of the cell. These results suggest that in the mouse ventricular myocardium, the shortening of the action potential during the late fetal period and the development of t-tubule-sarcoplasmic reticulum coupling during the second postnatal week largely contribute to the developmental increase in the dependence of contraction on sarcoplasmic reticulum function.

Electrical activity of the mouse pulmonary vein myocardium.

We recorded the electrical activity from the myocardial layer of isolated mouse pulmonary veins with the glass microelectrode technique. Spontaneous electrical activity was observed in about half of the preparations, which appeared either as constant firing or as repetitive bursts. Noradrenaline enhanced, while acetylcholine reduced, automatic activity. The action potentials evoked in quiescent preparations showed a resting membrane potential less negative than the atria and an extremely rapid early repolarization followed by a late plateau. The present study revealed that the mouse pulmonary vein myocardium shows diverse electrical activity, which is influenced by autonomic neurotransmitters.

A new statistical method for evaluation of L5178Ytk(+/-) mammalian cell mutation data using microwell method.

A statistical method to evaluate data from the mouse lymphoma L5178Y/tk assay (MLA) using microwell method is proposed. This proposed method is designed for data obtained from a single culture protocol instead of the duplicate culture recommended by United Kingdom Environmental Mutagen Society (UKEMS). The proposed method consists of the following three steps: (1) to apply Dunnett type test for identifying clear negative; (2) to apply a Simpson-Margolin procedure for detecting downturn data; and (3) to apply a trend test to evaluate the dose-dependent increase in mutant frequency (MF). The performance of the proposed method was evaluated through a Monte Carlo study and a case study. False positive rates realized in the Monte Carlo study were comparable with the UKEMS method modified for a single culture protocol with the heterogeneity factors being kept at 1.0. False negative rates were less than those of the modified UKEMS method for dose response patterns with a sharp uprise in higher dose groups, whereas, they were comparable for other patterns. The results of evaluating the data from an International Collaborative Study by the proposed method seem comparable with the UKEMS method. The proposed method enables us to evaluate data from the microwell MLA with a single culture protocol.