CARDIAC PROTEIN-CONTENT AND SYNTHESIS IN-VIVO AFTER VOLUNTARY RUNNINGOR HEAD-DOWN SUSPENSION

The adaptive responses of myocardial protein metabolism to chronic inc reases in work load were evaluated in juvenile female Sprague-Dawley r ats. Rats were studied under four conditions: normal weight bearing (N ), voluntary wheel running (WR) for less than or equal to 4 wk, head-d own-tilt suspension for 7 days (HS), or wheel running (2 or 3 wk) foll owed by 7 days of suspension (WR-HS). WR activity plateaued after 2 wk at 16 km/day and was maintained through week 4. WR did not affect nor mal whole body growth. Protein metabolism was studied by measuring hea rt protein content and in vivo fractional rate of protein synthesis wi th the [H-3]phenylalanine ''flooding dose'' method. Two weeks of WR in creased (P < 0.05) absolute heart protein content (22%) and protein sy nthesis (21%) relative to age-matched N group values. These difference s in protein content and synthesis were maintained for greater than or equal to 4 wk. Rats failed to gain significant body weight during sus pension. Heart protein content increased (P < 0.05) by 12% to 26% as d id protein synthesis (14% to 22%) in HS compared with N group. In WR-H S group, cardiac protein content and protein synthesis were maintained at significantly elevated levels. These findings indicate that 1) hig h-volume WR by young rats provides a convenient noninvasive method for producing rapid and substantial cardiac hypertrophy, which results, a t least in part, from enhanced cardiac protein synthesis; and 2) head- down suspension of sedentary juvenile rats leads to increased cardiac protein synthesis, which helps to increase cardiac protein content des pite a lack of whole body growth.