Muscle contraction under capillaries in hamster muscle induces arteriolar dilatation via K(ATP) channels and nitric oxide

Abstract

We tested the hypothesis that adenosine and nitric oxide can be sensed by capillaries and are implicated in the remote arteriolar dilatation initiated by muscle contraction. We also explored a role for K(ATP) channel activity in this response. Small bundles of muscle fibres underlying a group of capillaries in cremaster muscles of anaesthetized hamsters were electrically stimulated to contract for 2 min at each of 2, 4 and 8 Hz. Diameter changes were measured in the inflow arteriole to the group of capillaries after muscle contraction in the presence or absence of 10(-6) M xanthine amine congener (XAC) to block A(1) and A(2) adenosine receptors, 10(-4) or 10(-3) M N(omega)-nitro-L-arginine (LNNA) to block nitric oxide production, or 10(-5) M glibenclamide to block K(ATP) channel activity. Dilatations were unchanged with XAC (3.0 +/- 0.5, 3.9 +/- 0.7 and 6.1 +/- 1.0 microm), but were significantly reduced with LNNA (to 1.8 +/- 0.6, 3.5 +/- 0.7 and 4.9 +/- 0.7 microm) or glibenclamide (to 0.4 +/- 0.3, 0.8 +/- 0.7 and 1.9 +/- 0.6 microm). Neither K(ATP) channel activity nor nitric oxide was required for transmission or manifestation of the dilator response. Thus, muscle contraction can be sensed by capillaries and the signalling mechanism for the ensuing remote dilatation depends on K(ATP) channel activity and on NO, but not adenosine. Local application of 10(-4) M adenosine, 10(-4) M sodium nitroprusside or 10(-5) M pinacidil directly to capillaries initiated remote arteriolar dilatations. Thus, capillaries can respond directly to known mediators of metabolic vasodilatation, but these signalling pathways are not invariably implicated in the response to muscle contraction.

Figure 1. Schematic (not to scale) to show the principal features of the experimental site in cremaster muscle of anaesthetized hamsters

Figure 2. Diameter changes of module inflow arterioles (n = 7) in the absence (control, open bars; recovery, hatched bars) or presence (filled bars) of the adenosine A₁ and A₂ receptor antagonist xanthine amine congener (XAC, 10⁻⁶m), in response to 2 min of remote muscle contraction under capillaries at 2, 4 and 8 Hz

Also shown are responses of module inflow arterioles when adenosine (ADO, 10⁻⁴m) was applied topically to capillary modules (n = 9), in the absence (open bar) or presence (filled bar) of XAC. Bars are means ± s.e.m. All changes during muscle contraction are significantly different from rest, but XAC is not different from controls at any stimulation frequency. * Significantly different from the control for that condition.

Figure 3. Diameter changes of module inflow arterioles in the absence (control, open bars; recovery, hatched bars) or presence (filled bars) of the nitric oxide synthase inhibitor N^ω-nitro-l-arginine (LNNA, 10⁻⁴ or 10⁻³m), in response to 2 min of remote muscle contraction under capillaries at 2, 4 and 8 Hz

A, addition of LNNA to the superfusate, (n = 9); B, local application of LNNA to the observation site via micropipette (n = 5). Bars are means ± s.e.m. * Significantly different from the control for that condition.

Figure 4. Diameter changes of module inflow arterioles (n = 7) in the absence (control, open bars; recovery, hatched bars) or presence (filled bars) of the K_ATP channel antagonist glibenclamide (10⁻⁵m), in response to 2 min of remote muscle contraction under capillaries at 2, 4 and 8 Hz

Bars are means ± s.e.m. * Significantly different from the control for that condition.

Figure 5. Diameter changes of module inflow arterioles in response to 2 min of remote muscle contraction under capillaries at 2, 4 and 8 Hz, in the absence (control, open bars; recovery, hatched bars) or presence (filled bars) of the K_ATP channel antagonist glibenclamide (10⁻⁵m), locally applied via micropipette to the transmission pathway (A, n = 5) or the upstream observation site (B, n = 5)

Bars are means ± s.e.m. Responses in the presence of glibenclamide are not different from controls.

Figure 6. Diameter changes of module inflow arterioles induced by local micropipette application of the K_ATP channel opener pinacidil (10⁻⁵m, open bars) are reversed by addition of glibenclamide (10⁻⁵m, filled bars) added either to the superfusate (n = 6) or locally via micropipette (n = 4)

Bars are means ± s.e.m. * Significantly different from the control for that condition.

Figure 7. Diameter changes of module inflow arterioles (n = 9) in the absence (control, open bars; recovery, hatched bars) or presence (filled bars) of the K_ATP channel antagonist glibenclamide (10⁻⁵m) together with the nitric oxide synthase inhibitor N^ω-nitro-l-arginine (LNNA, 10⁻⁴m), in response to 2 min of remote muscle contraction under capillaries at 2, 4 and 8 Hz

Bars are means ± s.e.m. * Significantly different from the control for that condition.