In vivo Ca²⁺ buffering capacity and microvascular oxygen pressures following muscle contractions in diabetic rat skeletal muscles: fiber-type specific effects

Abstract

([Ca 2ϩ ]i) homeostasis is impaired following muscle contractions. It is unclear to what degree this behavior is contingent upon fiber type and muscle oxygenation conditions. We tested the hypotheses that: 1) the rise in resting [Ca 2ϩ ]i evident in diabetic rat slow-twitch (type I) muscle would be exacerbated in fast-twitch (type II) muscle following contraction; and 2) these elevated [Ca 2ϩ ]i levels would relate to derangement of microvascular partial pressure of oxygen (PmvO 2 ) rather than sarcoplasmic reticulum dysfunction per se. Adult male Wistar rats were divided randomly into diabetic (DIA: streptozotocin ip) 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 2ϩ imaging was achieved using fura 2-AM in vivo (i.e., circulation intact). DIA increased fatigability in EDL (P Ͻ 0.05) but not SOL. In recovery, SOL [Ca 2ϩ ]i either returned to its resting baseline within 150 s (CONT 1.00 Ϯ 0.02 at 600 s) or was not elevated in recovery at all (DIA 1.03 Ϯ 0.02 at 600 s, P Ͼ 0.05). In recovery, EDL CONT [Ca 2ϩ ]i also decreased to values not different from baseline (1.06 Ϯ 0.01, P Ͼ 0.05) at 600 s. In marked contrast, EDL DIA [Ca 2ϩ ]i remained elevated for the entire recovery period (i.e., 1.23 Ϯ 0.03 at 600 s, P Ͻ 0.05). The inability of [Ca 2ϩ ]i to return to baseline in EDL DIA was not associated with any reduction of SR Ca 2ϩ -ATPase (SERCA) 1 or SERCA2 protein levels (both increased 30 -40%, P Ͻ 0.05). However, Pmv O 2 recovery kinetics were markedly slowed in EDL such that mean Pmv O 2 was substantially depressed (CONT 27.9 Ϯ 2.0 vs. DIA 18.4 Ϯ 2.0 Torr, P Ͻ 0.05), and this behavior was associated with the elevated [ ] i following contraction in Type 1 diabetes, which remain to be resolved.The greater physiological fragility of fast-twitch fibers under atrophic conditions such as diabetes may relate, in part, to impaired microvascular structure (47) and hemodynamics (26). These effects impact the fine balance of O 2 delivery-to-O 2 utilization, at least within the spinotrapezius muscle, such that very low Pmv O 2 values are evident at rest and both during and following contractions (6,37,38). It is conceivable that the ϳ60% fast-twitch fibers that comprise the spinotrapezius (13) are driving these aberrant Pmv O 2 profiles.Predicated on the evidence presented above that SERCA activity is preserved/increased in isolated intact (or skinned) single muscle fibers in diabetes, we rationalized that impaired [Ca 2ϩ ] i homeostasis may relate to impaired microcirculatory hemodynamics (i.e., oxygenation, Pmv O 2 ) during/following contractions.

Release of skeletal muscle peptide fragments identifies individual proteins degraded during insulin deprivation in type 1 diabetic humans and mice

“…Higher LMW fragments from this protein isoform indicate a potential detrimental impact of insulin deprivation on myofibrillar integrity. The SR/ER Ca ATPase plays an important role in calcium transport from cytosol to the sarcoplasmic reticulum, and the increased degradation of this protein during insulin deprivation potentially affects its important function of the muscle in diabetes (13). Current approaches to measure protein degradation include amino acid tracers for whole body assessment, which can be combined with arterial-venous balance studies across a specific tissue bed (35) but cannot measure degradation of individual proteins or organelles such as mitochondria.…”

“…5) to investigate skeletal muscle protein turnover during insulin treatment (INSϩ) or insulin deprivation (INSϪ). Protein turnover kinetics were measured using infusion of [ring- 13 C 6 ]phenylalanine, providing a measure of whole body protein degradation (by dilution of plasma isotope from release of unlabeled amino acids). Plasma [ring- 13 C 6 ]phenylalanine isotopic enrichment at steady state was lower during INSϪ than during INSϩ due to a higher rate of appearance of unlabeled phenylalanine from whole body protein degradation (Fig.…”

Single muscle fiber contractile properties in diabetic RAT muscle

“…Indirect evidence that supports this suggestion is the recent finding that other functional properties, such as recovery of calcium homeostasis after muscle actions, are impaired in diabetic rat extensor digitorum longus but not in soleus muscle after 4 weeks of uncontrolled diabetes. 11 We found a significant reduction in absolute power (defined as force 3 velocity) in diabetic rat soleus muscle fibers. This is probably due to the observed reduction in force generating capacity, because no significant changes were seen in maximal shortening velocity.…”

“…Finally, it is important to note that changes in muscle quality may be fiber‐type specific and only seen in fibers expressing type II MHC but not type I MHC isoforms like in the rat soleus studied here. Indirect evidence that supports this suggestion is the recent finding that other functional properties, such as recovery of calcium homeostasis after muscle actions, are impaired in diabetic rat extensor digitorum longus but not in soleus muscle after 4 weeks of uncontrolled diabetes …”

“…Although muscle weakness and atrophy in diabetic patients can be explained par-tially by a neuropathic process, 10 there is evidence in both humans and animals to suggest independent intrinsic abnormalities at the level of individual muscle fibers. 2,3,7,11,12 Thus, it is possible that changes in muscle protein structure and/or myofilament interaction contribute to muscle dysfunction in diabetics.…”