Identification of CaV channel types expressed in muscle afferent neurons

Cardiovascular adjustments to exercise are partially mediated by group III/IV (small to medium) muscle afferents comprising the exercise pressor reflex (EPR). However, this reflex can be inappropriately activated in disease states (e.g., peripheral vascular disease), leading to increased risk of myocardial infarction. Here we investigate the voltage-dependent calcium (Ca-V) channels expressed in small to medium muscle afferent neurons as a first step toward determining their potential role in controlling the EPR. Using specific blockers and 5 mM Ba2+ as the charge carrier, we found the major calcium channel types to be Ca(V)2.2 (N-type) > Ca(V)2.1 (P/Q-type) > Ca(V)1.2 (L-type). Surprisingly, the Ca(V)2.3 channel (R-type) blocker SNX482 was without effect. However, R-type currents are more prominent when recorded in Ca2+ (Liang and Elmslie 2001). We reexamined the channel types using 10 mM Ca2+ as the charge carrier, but results were similar to those in Ba2+. SNX482 was without effect even though similar to 27% of the current was blocker insensitive. Using multiple methods, we demonstrate that Ca(V)2.3 channels are functionally expressed in muscle afferent neurons. Finally, ATP is an important modulator of the EPR, and we examined the effect on Ca-V currents. ATP reduced Ca-V current primarily via G protein beta gamma-mediated inhibition of Ca(V)2.2 channels. We conclude that small to medium muscle afferent neurons primarily express Ca(V)2.2 > Ca(V)2.1 >= Ca(V)2.3 > Ca(V)1.2 channels. As with chronic pain, Ca(V)2.2 channel blockers may be useful in controlling inappropriate activation of the EPR.