In Vivo Calcium Regulation in Diabetic Skeletal Muscle: Fiber-Type Specific Effects

Abstract

ntroduction: Type 1 diabetes impairs Ca ²⁺ handling in many tissues such as cardiac muscle, platelets, kidney and liver. In skeletal muscle, the diabetic state increases resting intracellular Ca ²⁺ ([Ca ²⁺] _i) levels and induces activity of Ca ²⁺-dependent proteolytic pathways. In skeletal muscle, dysfunctional contractile activity has been linked to impaired [Ca ²⁺] _i regulation. Muscle force production is impaired and fatigability and muscle fragility deteriorate with diabetes. Recently, we succeeded in measuring in vivo [Ca ²⁺] _i within the mixed fiber-type rat spinotrapezius muscle ¹⁾ following contractions. In this preparation, intramyocyte injection of a high [Ca ²⁺] bolus revealed a depressed Ca ²⁺ buffering capability in diabetes which corresponded with the elevated post-contraction [Ca ²⁺] _i ²⁾. Unfortunately, that preparation did not permit resolution of between fiber type effects on the profile of [Ca ²⁺] _i following contraction in Type 1 diabetes which remain to be resolved. We tested the hypotheses that: 1. The rise in resting [Ca ²⁺] _i evident in diabetic rat slow-twitch muscle would be exacerbated in fast-twitch muscle following contraction. 2. These elevated [Ca ²⁺] _i levels would relate to derangement of microvascular O₂ pressures (PmvO₂) rather than sarcoplasmic reticulum (SR) dysfunction per se.

Methods: Adult male Wistar rats were divided randomly into diabetic (DIA: Streptozotocin i.p.) and healthy (CONT) groups. Four weeks later extensor digitorum longus (EDL, predominately type II fibers) and soleus (SOL, predominately type I fibers) muscle contractions were elicited by continuous electrical stimulation (120 s, 100 Hz). Ca ²⁺ imaging was achieved using Fura-2 AM in vivo. Phosphorescence quenching techniques were used to measure PmvO₂.

Results: DIA increased fatigability in EDL (p<0.05) but not SOL. In recovery, SOL [Ca ²⁺] _i either returned to its resting baseline within 150 s (CONT, 1.00±0.02 at 600 s) or was not elevated in recovery at all (DIA, 1.03±0.02 at 600 s, p>0.05). In recovery, EDL CONT [Ca ²⁺] _i also decreased to values not different from baseline (1.06±0.01, p>0.05) at 600 s. In marked contrast, EDL DIA [Ca ²⁺] _i remained elevated for the entire recovery period (i.e., 1.23±0.03 at 600 s, p<0.05). The inability of [Ca ²⁺] _i 40 to return to baseline in EDL DIA was not associated with any reduction of SERCA1 or SERCA2 protein levels (both increased 30-40%, p<0.05). However, PmvO₂ recovery kinetics were markedly slowed in EDL such that mean PmvO₂ was substantially depressed (CONT, 27.9±2.0 vs DIA, 18.4±2.0 mmHg, p<0.05) and this behavior was associated with the elevated [Ca ²⁺] _i. In contrast, this was not the case for SOL (p>0.05) in that neither [Ca ²⁺] _i nor PmvO₂ were deranged in recovery with DIA.

Conclusion: In conclusion, compromised Ca ²⁺ buffering and elevated [Ca ²⁺] _i following an in vivo fatiguing tetanic contraction occur preferentially in the fast twitch EDL rather than the slow twitch SOL muscle. (The rest of omitted)