No abstract

Inward rectifier K channels (K) may contribute to skeletal muscle blood flow regulation and adapt to advanced age. Using mouse abdominal wall superior epigastric arteries (SEAs) from either young (3-6 mo) or old (24-26 mo) male C57BL/6 mice, we investigated whether SEA smooth muscle cells (SMCs) express functional K channels and how aging may affect K function. Freshly dissected SEAs were either enzymatically dissociated to isolate SMCs for electrophysiological recording (perforated patch) and mRNA expression or used intact for pressure myography. With 5 mM extracellular K concentration ([K]), exposure of SMCs to the K blocker Ba (100 μM) had no significant effect ( > 0.05) on whole cell currents elicited by membrane potentials spanning -120 to -30 mV. Raising [K] to 15 mM activated Ba-sensitive K currents between -120 and -30 mV, which were greater in SMCs from old mice than in SMCs from young mice ( < 0.05). Pressure myography of SEAs revealed that while aging decreased maximum vessel diameter by ~8% ( < 0.05), it had no significant effect on resting diameter, myogenic tone, dilation to 15 mM [K], Ba-induced constriction in 5 mM [K], or constriction induced by 15 mM [K] in the presence of Ba ( > 0.05). Quantitative RT-PCR revealed SMC expression of K2.1 and K2.2 mRNA that was not affected by age. Barium-induced constriction of SEAs from young and old mice suggests an integral role for K in regulating resting membrane potential and vasomotor tone. Increased functional expression of K channels during advanced age may compensate for other age-related changes in SEA function. Ion channels are integral to blood flow regulation. We found greater functional expression of inward rectifying K channels in smooth muscle cells of resistance arteries of mouse skeletal muscle with advanced age. This adaptation to aging may contribute to the maintenance of vasomotor tone and blood flow regulation during exercise.

T‐type voltage‐gated Ca2+ channels (CaV3.2 VGCC) have been hypothesized to control spontaneous transient outward currents (STOCs) through large‐conductance Ca2+‐activated K+ channels (BKCa), and contribute to the negative‐feedback regulation of myogenic tone. We tested this hypothesis in superior epigastric arteries (SEAs) isolated from male C57BL/6 mice. SEAs were isolated and enzymatically dissociated to obtain single smooth muscle cells (SMCs) for whole‐cell recording of paxilline‐sensitive (PAX, 1 μmol/L) STOCs at −30 mV, or cannulated and studied by pressure myography (80 cm H2O, 37°C). The CaV3.2 blocker Ni2+ (30 μmol/L) had no effect on STOC amplitude (20.1 ± 1.7 pA vs. 20.6 ± 1.7 pA; n = 12, P = 0.6), but increased STOC frequency (0.79 ± 0.15 Hz vs. 1.21 ± 0.22 Hz; n = 12, P = 0.02). Although Ni2+ produced concentration‐dependent constriction of isolated, pressurized SEAs (logEC50 = −5.8 ± 0.09; E max = 72 ± 5% constriction), block of BKCa with PAX had no effect on vasoconstriction induced by 30 μmol/L Ni2+ (in the absence of PAX = 66 ± 4% constriction vs. in the presence of 1 μmol/L PAX = 65 ± 4% constriction; n = 7, P = 0.06). In contrast to Ni2+, the nonselective T‐type blocker, mibefradil, produced only vasodilation (logEC50 = −6.9 ± 0.2; E max = 74 ± 8% dilation), whereas the putative T‐type blocker, ML218, had no significant effect on myogenic tone between 10 nmol/L and 10 μmol/L (n = 6–7, P = 0.59). Our data do not support a role for CaV3.2 VGCC in the negative‐feedback regulation of myogenic tone in murine SEAs and suggest that Ni2+ may constrict SEAs by means other than block of CaV3.2 VGCC.

Elevating extracellular K+ ([K+]o) dilates resistance arteries by activating smooth muscle cell (SMC) inward‐rectifier K+ channels (KIR). The rise in [K+]o of exercising skeletal muscle can thereby contribute to functional hyperemia. As muscle blood flow is diminished with aging, we tested the hypotheses that aging impairs K+‐induced activation of KIR in SMCs of resistance arteries. Superior epigastric arteries (SEAs) were dissected from abdominal skeletal muscle of Young (Y; 蠅3 mo) or Old (O; 蠅24 mo) male C57BL/6 mice and either dissociated to yield SMCs for perforated patch clamp recording or cannulated and pressurized for myography. Elevation of [K+]o from 5 to 15 mM activated Ba2+ (100 μM)‐sensitive inward K+ currents between ‐120 and ‐60 mV that were smaller in SMCs from Y than in O: at ‐120 mV, KIR current density was ‐0.2±0.3 pA/pF in Y vs ‐0.8±0.2 pA/pF in O; n=26; p<0.05. In contrast, elevating [K+]o from 5 to 15 mM evoked similar dilation in SEAs of Y (61±6%; n=12) and O (60±7%; n=13); responses in both were abolished by 100 μM [Ba2+]o (n=5). Thus, aging does not impair KIR currents in SMCs of resistance arteries and their dilation via KIR activation is maintained. These findings indicate that compensatory adaptations with aging maintain SEA function in the presence of increased KIR activity. Grant Funding Source: Supported by NIH R01 HL086483 and R25 HL103156