Hypocaloric feeding (HCF) depresses heart function causing cardiac atrophy, bradycardia, and decreased cardiac output. We tested the hypothesis that HCF results in decreased myocardial Ca- and ATP cycling. We reduced protein-calorie intake of adult rats by 20% for 7 days and then allowed them to recover for 3 days. Changes in ionized Ca concentration (nM/s) of 2.5% myocardial homogenates that were attributable to the Ca-ATPase pump and Ca-release channel (CRC), respectively, of the sarcoplasmic reticulum (SR) were depressed 41 and 85% by HCF from 61.6 +/- 9.4 and 24.7 +/- 3.3, to 36.1 +/- 2.8 and 3.6 +/- 2.9. Activity of the Ca-pump was restored after 3 days of refeeding, whereas the CRC remained 23% depressed (all p < 0.05). Additionally, the CRC activity was inhibited to a 3-fold greater extent than controls by HCF, but was disinhibited within one day of refeeding. The greater effect on CRC than Ca-pump activity resulted in net Ca-uptake being unaffected by HCF. In addition to depression of Ca-cycling, ATP sythetase and total ATPase activities (IU/g), respectively, were depressed 20 and 15% by HCF from 174 +/- 19 and 51.3 +/- 3.8 to 140 +/- 15 and 43.7 +/- 4.7, but were restored to control values within one day of refeeding. We conclude that HCF produces a compensatory, reversible, and asymmetric downregulation and inhibition of Ca-cycling, with the CRC being preferentially affected.