Howell, Sandra, Wen-Zhi Zhan, and Gary C. Sieck. Diaphragm disuse reduces Ca²⁺ uptake capacity of sarcoplasmic reticulum. J. Appl. Physiol. 82(1): 164-171, 1997.Chronic phrenic tetrodotoxin (TTX) blockade and phrenic denervation (Dnv) of hamster diaphragm result in decreased maximum specific tension, prolonged contraction time, and improved fatigue resistance (W. Z. Zhan and G. C. Sieck. J. Appl. Physiol. 72: 1445-1453, 1992). An underlying increased relative contribution of type I fibers to total muscle mass appears to be consistent with, but does not completely account for, changes in contractile and fatigue properties. The present study was designed to evaluate a potential role for altered cellular Ca²⁺ metabolism in the adaptive response of the diaphragm to chronic disuse. An analytic method based on simulation and modeling of long-term ⁴⁵Ca²⁺ efflux data was used to estimate Ca²⁺ contents (nmol Ca²⁺/g wet wt tissue) and exchange fluxes (nmol Ca^{2+ ·} min^{1 ·} g¹) for extracellular and intracellular compartments in the in vitro hamster hemidiaphragm after prolonged disuse. Three groups were compared: control (Con, n = 5), phrenic TTX blockade (TTX, n = 5), and phrenic denervation (Dnv, n = 5). Experimental muscles were loaded with ⁴⁵Ca²⁺ for 1 h, and efflux data were collected for 8 h by using a flow-through tissue chamber. Compartmental analysis of efflux data estimated that the Ca²⁺ contents and Ca²⁺ exchange fluxes of the largest and slowest intracellular compartment (putative longitudinal reticulum) were reduced by ~50% in TTX and Dnv muscle groups compared with Con. In addition, the kinetic model predicted significant decreases in total intracellular Ca²⁺ and total diaphragm Ca²⁺ in TTX and Dnv muscles. We conclude that the data support the hypothesis that the capac- ity of the sarcoplasmic reticulum for Ca²⁺ sequestration is reduced in chronic diaphragm disuse. The impact of this effect on diaphragm contractile and fatigue properties is discussed.