Sensitization of group lV skeletal muscle afferents in type 1 diabetic rats
Authors: Ishizawa Rie , Kim Han Kyul , Hotta Norio , Iwamoto Gary A. , Vongpatanasin Wanpen , Mitchell Jere H. , Smith Scott A. , Mizuno Masaki 
Published: April 2020

The blood pressure (BP) response to exercise is abnormally exaggerated in type 1 diabetes mellitus (T1DM). However, the mechanisms underlying the potentiated BP response remain largely unclear. Sensory information from working muscle, generated by activation of mechanically and metabolically sensitive muscle afferents, is known to contribute importantly to this exercise‐induced exaggeration in BP. Moreover, receptors for advanced glycation end products (RAGE) and protein kinase C (PKC) interact to modulate neuronal function, including muscle afferents. It is possible that the interactive relationship between RAGE and PKC is altered in T1DM. Therefore, it was hypothesized that the potentiated exercise‐induced BP response in T1DM is mediated by sensitization of skeletal muscle afferent fibers via an overactive RAGE/PKC signaling pathway.

PURPOSE

To investigate 1) action potential responses to mechanical and chemical stimulation in muscle afferents of healthy and T1DM rats in vitro, 2) the impact of T1DM on plasma levels of high‐mobility group box‐protein 1 (HMGB‐1), a RAGE ligand, and 3) the impact of T1DM on PKC activation in dorsal root ganglion (DRG) subserving skeletal muscle afferent neurons.

METHODS

T1DM was induced in Sprague‐Dawley rats by intraperitoneally injecting a single dose of 50 mg/kg streptozotocin. In healthy control animals, only saline was injected. At 1–3 weeks after the injection, the function of mechanically (application of a 0–196 mN ramped stimulus) or chemically (administration of 1 μM capsaicin) activated group IV fibers were assessed by obtaining single‐fiber recordings using the extensor digitorum longus muscle‐nerve preparation. Plasma HMGB1 was determined using ELISA. Phosphorylated PKC protein levels were quantified in DRG (L3‐L5) by western blotting.

RESULTS

T1DM rats exhibited hyperglycemia after overnight fasting (P<0.05). Plasma insulin in T1DM tended to be lower as compared to control animals (P=0.09). Spontaneous activity in group IV fibers was significantly larger in diabetic rats than in control animals (P<0.05). Compared to control, response magnitudes to mechanical stimulation (P<0.01) and capsaicin exposure (P=0.01) in group IV afferents were significantly greater in T1DM. Plasma HMGB1 levels were significantly higher in T1DM as compared with control (P<0.01). T1DM showed markedly increased phosphorylated PKC levels in DRGs compared with control (P<0.05).

CONCLUSIONS

In T1DM, the sensitivity of group IV skeletal muscle afferent fibers to mechanical and chemical stimuli is abnormally increased. The data likewise suggest this enhanced afferent sensitivity may be due to augmented activation of the RAGE/PKC pathway. Importantly, these changes likely contribute significantly to the exaggerated pressor response to exercise characteristic of T1DM.

Support or Funding Information

Supported by Lawson & Rogers Lacy Research Fund and SHP Interdisciplinary Research Grant Program.