Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia

Abstract

Responses evoked in muscle sympathetic nerve activity (MSNA) by systemic hypoxia have received relatively little attention. Moreover, MSNA is generally identified from firing characteristics in fibres supplying whole limbs: their actual destination is not determined. We aimed to address these limitations by using a novel preparation of spinotrapezius muscle in anaesthetised rats. By using focal recording electrodes, multi-unit and discriminated single unit activity were recorded from the surface of arterial vessels.This had cardiac- and respiratory-related activities expected of MSNA, and was increased by baroreceptor unloading, decreased by baroreceptor stimulation and abolished by autonomic ganglion blockade. Progressive, graded hypoxia (breathing sequentially 12, 10, 8% O2 for 2min each) evoked graded increases in MSNA.In single units, mean firing frequency increased from 0.2±0.04 in 21% O2 to 0.62 ± 0.14 Hz in8% O2, while instantaneous frequencies ranged from 0.04–6Hz in 21% O2 to 0.09–20 Hz in 8%O2. Concomitantly, arterial pressure (ABP), fell and heart rate (HR) and respiratory frequency(RF) increased progressively, while spinotrapezius vascular resistance (SVR) decreased (Spinotrapezius blood flow/ABP), indicating muscle vasodilatation. During 8% O2 for 10 min, the falls in ABP and SVR were maintained, but RF, HR and MSNA waned towards baselines from the second to the tenth minute. Thus, we directly show that MSNA increases during systemic hypoxia to an extent that is mainly determined by the increases in peripheral chemoreceptor stimulation and respiratory drive, but its vasoconstrictor effects on muscle vasculature are largely blunted by local dilator influences, despite high instantaneous frequencies in single fibres.

Figure 1. Nerve activity recorded from the surface of an arterial vessel of spinotrapezius under normoxic conditions

A, raw multi-unit nerve activity. MSNA, respiratory air –flow; EXP, expiration; INS, inspiration; ABP, arterial pressure. B and C, examples of ABP- and tracheal pressure-triggered cross-correlation histograms showing the cardiac- and respiratory-related rhythmicities, respectively, of the multi-unit activity (histogram triggers: 480 and 144, respectively). D, multi-unit activity recorded from another animal under normoxic conditions. Below, 4 different examples of 4 or 5 superimposed action potentials discriminated from multi-unit activity.

Figure 4. Typical example of rhythms recorded in single unit activity in normoxia (21% O₂) over 1 min before and over the second 1 min of progressive graded levels of hypoxia (12, 10, 8% O₂)

A, systolic pressure-triggered cross-correlations (100 bins, width 0.01 s) shown above ABP recordings (histogram triggers: 348 and 363, 396, 480 from top down). B, expiratory-triggered cross correlations (100 bins, width 0.05 s) shown above recordings of respiratory airflow (histogram triggers: 96 and 114, 116, 120 from top down). NB, respiratory and cardiac-related rhythmicities present during each inspirate, but cardiac-related and respiratory-related rhythmicity became less, and more distinct, respectively, in severe hypoxia.

Figure 2. Responses evoked in MSNA by bolus injection of sodium nitroprusside (SNP)

Original recordings above: MSNA (analysed as spikes s⁻¹), heart rate (HR; in beats min⁻¹, b.p.m.), raw multi-unit activity and arterial pressure (ABP). Histograms below show mean ± SEM of mean ABP (mABP), multi-unit activity and single unit activity discriminated from multi-unit activity, over three 10 s periods, before (baseline), at the peak of the response evoked by SNP (SNP) and in recovery. ***P < 0.001, baseline vs. SNP (n = 6 for number of rats and for number of single units, i.e. one was analysed in each animal).

Figure 3. Respiratory and cardiovascular responses evoked by progressive graded hypoxia

A, original recordings showing responses evoked by successive 2 min periods of breathing 12, 10 and 8% O₂ beginning at arrows; downward arrow indicates return to 21% O₂. B, mean systemic cardiovascular and respiratory responses and changes in MSNA evoked by progressive, graded hypoxia. Values are shown as means ± SEM in final 1 min before, and in the second 1 min at each level of hypoxia and after return to 21% O₂. R_F, respiratory frequency; other abbreviations as in Fig. 2. ***P < 0.001, **P < 0.01, *P < 0.05, for 12, 10 or 8% O₂vs. 21% O₂ by post hoc analysis (n = 6). C, original recordings showing changes in MSNA and ABP associated with 2 augmented breaths (•): additional inspiratory effort at end of normal inspiration. NB, MSNA ceases before augmented breath.

Figure 5. Muscle blood flow and vascular resistance changes evoked by progressive, graded hypoxia

A, original recordings of with the addition of spinotrapezius and femoral blood flow (SBF and FBF, respectively) and spinotrapezius and femoral vascular resistance (SVR and FVR, respectively) computed as ABP/SBF or FBF. Other abbreviations, as in Fig. 1. Arrows indicate onset of each inspirate for 2 min each. B, mean systemic cardiovascular, respiratory and regional vascular responses evoked by progressive, graded hypoxia. Values are shown as means ± SEM in final 1 min before, and in the second 1 min at each level of hypoxia and after return to 21% O₂. ***P < 0.001, **P < 0.01, *P < 0.05, for 12, 10 or 8% O₂vs. 21% O₂ by post hoc analysis (n = 10).

Figure 6. Mean systemic cardiovascular, respiratory and femoral vascular responses and changes in MSNA evoked by 10 min period of breathing 8% O₂

Abbreviations as in Figs 1 and 5. Values are shown as means ± SEM in final 1 min before, in the 2nd and 9th min of hypoxia and in the 2nd min after return to 21% O₂. ***P < 0.001, **P < 0.01, *P < 0.05, for 8% O₂vs. 21% O₂. †P < 0.05, 2nd min vs. 9th min (n = 4).