Based upon ultrastructural considerations it has been postulated that Ca 2+ flux into cardiac cells will not pass directly to the myofilaments but rather is sequestered by sarcoplasmic reticulum (SR) opposed between the sarcolemma and myofilaments. Direct proof of the hypothesis is lacking. We tested the hypothesis that the SR buffers transsarcolemmal Ca 2+ influx during the action potential by devising experiments that deplete the SR of Ca 2+ and thus prevent the large immediate SR Ca 2+ release into the cytosolic space following excitation. This SR Ca 2+ depletion was accomplished by two methods. In the first method caffeine which elicits a large and rapid release of Ca 2+ stored within the SR is pulsed onto rested guinea pig myocytes. A subsequent rapid application of caffeine to SR depleted cells fails to elicit Ca 2+ release. Thus, the increase in cytosolic Ca 2+ (Ca i) following electrical stimulation of such cells cannot be immediately derived from the SR; measurements indicate that its onset is delayed, its rate of increase is slowed compared to non SR depleted cells. A caffeine spritz given simultaneously with the onset of depolarization during an action potential, however, elicits a large, rapid increase Ca i. This indicates that the SR rapidly loads with Ca 2+ during the depolarization and that a large fraction of this Ca 2+ load is not immediately available to the cytosol or to the myofilaments. In the second model using thapsigargin, a purported SR pump inhibitor, the Cai transient and contraction of rested guinea pig cells is more rapid and larger in amplitude than prior to thapsigargin, permitting the same conclusion as the caffeine depletion model i.e. rapid SR Ca sequestration of Ca 2+ influx occurs during the action potential. GRANT=Z01AG00228 A. The effect of age, gender and conditioning status on heart rate variability at rest and during exercise are being assessed in conjunction with investigators from the University of Maryland College Park. Variations in electrocardiographic R-R intervals are determined using a polynomial averaging algorithm at supine rest, during quiet sitting and standing, during graded treadmill exercise and during recovery from exercise. Preliminary results at rest demonstrate the anticipated age- associated decline in R-R variability but identify a subset of fit elderly men with R-R variability comparable to that of young men. B. The prognostic significance of 24-hr ambulatory ECG recordings was assessed in 100 healthy BLSA volunteers > 60 years old. Over a mean follow up of 10 years, coronary events (CE) developed in 10 subjects. The prevalence and complexity of both supraventricular (SV) and ventricular (V) ectopic beats were similar in the groups with and without CE. However, CE occurred in 2 of 5 subjects (40%) with flat or downsloping ST segment depression > 1.0 mm versus only 8 of 95 (8%) without such ST changes. C. Longitudinal change in 24 hour ambulatory ECG recordings of healthy BLSA volunteers initially aged greater than or equal to 60 years is being assessed by repeat 24 hour ECG recording a minimum of 10 years after initial recording. Thus far follow-up recordings a minimum of 10 years after initial recording. Thus far follow-up recordings have been made in 35 subjects; results are pending.
