Mathematical analysis of type-I and type-IIb muscle fiber force generation in renal hypertension.

Previous results from our laboratory have shown that isometric tension development is significantly lower in reduced renal mass (RRM) hypertensive rats when compared to sham-operated controls. The current study was designed to mathematically analyze isometric tetanic contraction profiles and determine the relative contribution of fast-glycolytic (FG) and slow-oxidative (SO) muscle fibers produced by the isolated gastrocnemius-plantaris-soleus muscle group of RRM and sham rats. Because renal hypertension has been shown to be associated with a reduction in microvascular density, we hypothesized that renal hypertension leads to a decrease in SO muscle fiber contribution to force generation. The mathematical model determined the force contribution of two independent muscle fiber components, SO and FG, to the contraction and relaxation phase of isometric tetanic contractions. Each phase was modeled as having an exponentially rising contraction phase during the stimulus period and an exponentially decaying relaxation phase when the stimulus was removed. Each fiber type's tension was also scaled by an envelope function describing the fatigue over the contraction bout. The model, which included 10 parameters, was fit to experimental data by using a nonlinear optimization method and described certain limited characteristics of both fiber types. Results from this model suggest that renal hypertension affects skeletal muscle force generation primarily by decreasing the SO muscle fiber contribution to the total developed tension, decreasing performance and increasing muscle fatigue in RRM rats.