Comparative studies on microinjected high-mobility-group chromosomal proteins, HMG1 and HMG2

The nonhistone chromosomal proteins, HMG1 and HMG2, were iodinated and introduced into HeLa cells, bovine fibroblasts, or mouse 3T3 cells by erythrocyte-mediated microinjection. Autoradiographic analysis of injected cells fixed with glutaraldehyde consistently showed both molecules concentrated within nuclei. Fixation with methanol, on the other hand, resulted in some leakage of the microinjected proteins from the nuclei so that more autoradiographic grains appeared over the cytoplasm or outside the cells. Both injected and endogenous HMG1 and HMG2 partitioned unexpectedly upon fractionation of bovine fibroblasts, HeLa, or 3T3 cells, appearing in the cytoplasmic fractions. However, in calf thymus, HMG1 and HMG2 molecules appeared in the 0.35 M NaCl extract of isolated nuclei, as expected. These observations show that the binding of HMG1 and HMG2 to chromatin differs among cell types or that other tissue-specific components can influence their binding. Coinjection of [125I]HMG1 and [131I]HMG2 into HeLa cells revealed that the two molecules display virtually equivalent distributions upon cell fractionation, identical stability, identical intracellular distributions, and equal rates of equilibration between nuclei. In addition, HMG1 and HMG2 did not differ in their partitioning upon fractionation nor in their stability in growing vs. nongrowing 3T3 cells. Thus, we have not detected any significant differences in the intracellular behavior of HMG1 and HMG2 after microinjection into human, bovine, or murine cells.

The C-wave of the rabbit electroretinogram during dark-adaptation and the steady-state

C-waves of the rabbit ERF under urethane anesthesia were elicited by a Grass photostimulator during dark adaptation and under steady-state conditions. After strong pre-adaptation with light, the amplitude and time-to-peak of the c-wave increased in parallel with the a- and b-waves, reaching a maximum of 90-130 min. In the dark-adapted steady-state, repetitive stimuli at intervals ranging from 15 sec to 5 min elicited a stable c-wave response except for a small dip in amplitude that occurred 2-6 min after initiating the shortest interval (15 sec) flashes. Although the changes in light and dark used in these experiments should have elicited shifts in the standing potential, no fluctuations of the c-wave amplitude were evident with a time course corresponding to a light response of the standing potential.

A prospective study of mitral valvular prolapse in young men

A cardiac history, a physical examination, an electrocardiogram, phonocardiograms in the supine and standing positions, and an M-mode echocardiogram were obtained in 100 randomly selected, presumably healthy, male medical students (mean age, 26 years). Four percent met standard echocardiographic criteria for mitral valvular prolapse. No midsystolic clicks or late systolic murmurs were appreciated in this group, and none complained of chest pain or palpitations. To elucidate further the clinical implication of the echocardiographic pattern of mitral valvular prolapse, 24-hour ambulatory ECGs, multistage exercise tests, and scintiscans of myocardial perfusion at rest and after exercise (using radioactive 13nitrogen-labelled ammonium) were obtained, with normal results. The absence of life-threatening arrhythmias and exercise-induced abnormalities in these four asymptomatic subjects without abnormal physical findings suggests that the echocardiographic pattern of mitral valvular prolapse in such individuals may represent a variant of normal which does not require extensive evaluation.