Decrease in heart peptide initiation during head-down tilt may be modulated by HSP-70

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Abstract
This study examines the mechanism of the rapid decrease in cardiac muscle protein synthesis during rodent hindlimb non-weight bearing. Polysomes isolated from rat hearts 8 h after suspension show less RNA in the polysome pool and a shift in polysome size toward fewer ribosomes per mRNA; 18 h after suspension, the size shift persists, but the amount of RNA in the polysome pool returns to control values. These data are consistent with a decrease in the rate of initiation of protein synthesis. At both 8 and 12 h of suspension, the cardiac polysomes show a 78 and 93% increase association with the nascent polypeptide chaperone protein 70-kDa heat-shock cognate/heat-shock protein (HSC/HSP-70), respectively, that persists after 7 days of non-weight bearing. Because the dissociation of HSC/HSP-70 from unfolded protein can be modulated by ATP, we measured the adenosine nucleotide pools and found a 53% decrease in ATP levels after 18 h of suspension. We propose a mechanism in which a shift of HSC/HSP-70 to the nascent polypeptide indirectly inhibits protein synthesis initiation.