Relative significance of the nitric oxide (NO)/cGMP pathway and K+ channel activation in endothelium-dependent vasodilation in the femoral artery of developing piglets.

Mechanisms mediating endothelium-dependent vasodilation were investigated in femoral artery rings from <2-day-old (newborn) and 2-week-old piglets. Based on previous results we hypothesized an age difference in the relative contribution of nitric oxide(NO)-cyclic 3',5'-guanosine monophosphate (cGMP) and K+ channel-activation to acetylcholine (ACh)-induced vasodilation. Changes in vascular tone were studied in organ baths in the absence or presence of NO synthase(NOS) inhibition or K+ channel blockade and the intra-arterial accumulation of cGMP in response to ACh was measured with radioimmunoassay (RIA). In control experiments, relaxant responses to ACh were equal in the two age groups. In the presence of the NOS-inhibitors N G-monomethyl-L-arginine acetate (L-NMMA; 100 microM) or NG-nitro-L-arginine (L-NOARG; 1-100 microM), however, relaxation was significantly more reduced in femoral artery rings from 2-week-old than from newborn, with lower pD2 values in the older age group. Inhibition of large (BKCa) conductance calcium-sensitive K+ channels with tetraethylammonium chloride (TEA; 1 mM), gave a significant rightward shift in the concentration-response curves to ACh which was of the same magnitude in both age groups. The ACh-induced vasodilation was abolished in both age groups by high K+ (20 mM) in combination with L-NOARG (100 microM). The relative increase in cGMP levels after addition of ACh (10 nM) was significantly larger in rings from newborn compared with 2-week-old piglets (12- vs. four-fold). In summary, sensitivity to NOS inhibition increased with age while the effect of K+ channel blockade with TEA was the same in femoral artery rings from newborn to 2-week-old piglets. Lower sensitivity to NOS inhibition and a larger increase in cGMP in response to ACh could indicate a higher efficacy of the NO/cGMP pathway in this vessel in the newborn piglet.

Heart rate response to transient chemoreceptor stimulation in term infants is modified by exposure to maternal smoking.

Modulation of heart rate (HR) during transient hyperoxia, hypoxia, and hypercapnia was studied in 46 healthy term infants on 103 occasions (postnatal d 2 to 82). Twenty-three infants had smoking mothers (median, 11 cigarettes/d). Transient chemoreceptor stimuli (100% O(2), 15% O(2), or 3% CO(2)) were presented repeatedly during quiet sleep. Beat-by-beat HR and breath-by-breath ventilation were recorded continuously. The coherently averaged HR and ventilation responses to each stimulus were calculated for each infant at each age. Outcome variables (HR change from baseline to end of stimulation, maximum HR change, and time to half-maximum) were analyzed by ANOVA. Overall, HR declined during hyperoxia (median change, 4.2 beats/min) and rose during hypoxia (median change, 4.2 beats/min) and hypercapnia (median change, 4.6 beats/min). The percentage change in HR was positively correlated with the percentage change in ventilation (p < 0.001). Increasing number of cigarettes smoked by the mother was correlated with deeper HR declines and smaller HR rises (p = 0.02). For the population as a whole, the HR response lagged 3.8 s behind the ventilatory response during hyperoxia and hypoxia (p < 0.001), whereas during hypercapnia there was no significant lag. The lag in HR response in the smoke-exposed group was 2.5 s greater than that in the control group for all three stimuli (p = 0.001), and the difference increased with the number of cigarettes smoked by the mother (p < 0.01). Both pulmonary reflexes and the type of the chemoreceptor stimulus seemed to influence HR. Maternal smoking affected the magnitude and time-course of the HR response in a dose-dependent manner.

A method of assessing ventilatory responses to chemoreceptor stimulation in infants.

Various methods of assessing infant chemoreceptor responses have been reported in the literature. However, equipment dead space, trigeminal stimulation and inherent respiratory variability may have affected the results. A method is presented which attempts to reduce the effect of these factors and thereby isolate the chemoreceptor response. Inspiratory gas was delivered into a lightweight face mask with a pliable rim, minimal dead space and a connected pneumotachograph. Ventilatory data were computed breath by breath. Computer-controlled electromagnetic valves allowed instantaneous switching between air and different gas mixtures, repeated in a randomized sequence. In 18 healthy term neonates, the mask increased ventilation by 12% (95% confidence interval 6-18%), measured by calibrated strain-gauge bands. The effect on respiratory frequency and tidal volume differed significantly between sleep states. Neonates were challenged with short-lasting hyperoxia, mild hypoxia, rebreathing and mild hypercapnia. Coherent averaging of several ventilatory responses from each sleep state reduced the variability while maintaining a high time-resolution.

Development of oxygen sensitivity in infants of smoking and non-smoking mothers.

AIMS: To assess the effect of prenatal cigarette smoke exposure on the postnatal resetting of oxygen sensitivity in term infants. METHODS: 15 healthy term infants of smoking mothers (median 10 cigarettes/day) and 16 controls were studied during quiet sleep 1, 3, and 10 days and 10 weeks postnatally. Strain-gauge respiratory trace was continuously recorded. Repeated 15-s challenges with 100% O2 and 15% O2 were presented in randomised order through a face mask. A median of six hyperoxic and six hypoxic challenges per recording were obtained. Breath-by-breath ventilation in a time-window from 20 s before onset of stimulus to 60 s after was extracted. For each infant at each age, the normalised coherently averaged response to hyperoxia and hypoxia was calculated. Mean ventilation at end of the 15-s stimulus was analysed with ANOVA, as were parameters describing a function fitted to each averaged response. RESULTS: During air breathing, smoke-exposed infants had higher respiratory rates and lower tidal volumes than controls. Nicotine concentration in infant hair, measured by gas chromatography, was positively correlated with maternal level of smoking. A long-term development in oxygen sensitivity was demonstrated in both groups. However, neither the time-course nor the magnitude of O2 responses was affected by maternal smoking. Overall, hyperoxia reduced ventilation by 6.3% at day 1, 13.2% at day 3, 29.6% at day 10, and 40.0% at week 10. Transient hypoxia increased ventilation by 3.5%, 3.2%, 6.4%, and 8.8%, respectively, at the four ages studied.

Postnatal changes in mechanisms mediating acetylcholine-induced relaxation in piglet femoral arteries.

We studied the nitric oxide-cGMP pathway in endothelium-dependent relaxation in femoral arterial rings from piglets at different postnatal ages. Responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were examined in phenylephrine-precontracted rings from newborn (10-22-h) and 7 d (7-10-d)-old piglets. Relaxant responses were investigated in endothelium-denuded rings and endothelium-intact controls, and in endothelium-intact rings incubated with the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl-L-arginine acetate (L-NMMA), indomethacin, or the superoxide anion generator 6-anilinoquinoline-5,8-quinone (LY83583). Arterial rings from both age groups relaxed to a similar degree in response to ACh. Relaxation in rings from newborn piglets was insensitive to NOS inhibition by L-NMMA, whereas in artery rings from 7-d-old piglets, the relaxant response was significantly inhibited by L-NMMA. Incubation with LY83583 gave an inhibition of ACh-induced relaxation very similar to that of L-NMMA. Incubation with indomethacin had no significant effect on ACh-induced relaxation in either age group. Artery rings from both age groups relaxed 100% to SNP; the 7-d-old group was more sensitive than the newborn. NOS inhibition potentiated SNP-induced relaxation in both groups, but the potentiating effect was of greater magnitude in the newborn. Our results indicate a difference in the mechanism(s) underlying ACh-induced relaxation in the femoral artery from newborn and 7-d-old piglets, with an intact relaxant response in rings from the newborn despite NOS inhibition. The SNP results indicate a down-regulated soluble guanylate cyclase in the newborn, possibly related to a difference in basal NO release between the two age groups.

Spontaneous flow waves detected by laser Doppler in human skin.

The contribution of spontaneous variations in sympathetic nervous activity and perfusion pressure to the laser Doppler flux signal in human skin was studied in nine healthy subjects. Simultaneous recordings were made of laser Doppler flux, mean blood pressure, and blood flow in the radial artery. In the skin of the palm and sole, there was a significant and strong correlation between fluctuations in flux and radial artery velocity, which indicated a high degree of neural control of microcirculatory blood flow. This correlation decreased progressively toward the trunk and face. In the skin of the nose and at scattered sites on the face, trunk, and extremities, there was correlation between flux and mean blood pressure that indicated a predominantly "passive" vascular bed. At other sites, multivariate regression analysis revealed contributions of both blood pressure and sympathetic activity. In skin on the ear and forehead, a characteristic pattern of regular oscillations in flux, uncorrelated with blood pressure and sympathetic activity, was demonstrated. The laser Doppler signal from different skin sites thus contains varying contributions from variations in perfusion pressure, sympathetic nervous activity, and local, myogenic arteriolar vasomotion.

A causal relationship between fluctuations in thermoregulatory skin perfusion and respiratory movements in man.

The relationship between regular respiration with normal tidal volume and spontaneous fluctuations in blood flow through skin arteriovenous anastomoses (AVAs) was investigated. Laser Doppler measurements from skin areas known to contain arteriovenous anastomoses, ultrasound Doppler measurements from the radial artery and respiratory movements were recorded simultaneously in 7 supine human subjects in a thermoneutral environment. The phase relationship between respiration and the onset of sudden arteriovenous anastomoses vasoconstrictions was calculated in each subject. A few seconds before a vasoconstriction, there was a clear tendency towards synchrony in the respiration recordings, indicating that some respiration-phase-dependent event precedes the vasoconstrictions. Since arteriovenous anastomoses constrictions are accompanied by specific changes in heart rate and blood pressure, our findings link respiration to low-frequency heart rate and blood-pressure variability in humans.

Connection between skin arteriovenous shunt flow fluctuations and heart rate variability in infants.

Large, spontaneous fluctuations in blood flow to acral skin, caused by synchronous opening and closing of arteriovenous anastomoses (AVAs), have been demonstrated in adults in a thermoneutral environment. Individual AVA constrictions were accompanied by a diphasic heart rate (HR) response, indicating the presence of an autonomic rhythm which affected both skin AVA activity and heart rate variability. In the present study, 24 neonates were examined on day 2 (range 1-3 days) and re-examined at 14 weeks (11-17 weeks). The presence of rhythmic, synchronous fluctuations in laser Doppler flux in the palm of the hand and sole of the foot in 20 of 24 neonates strongly indicated the presence of functional skin AVA at birth. Both neonates and 3-month-old infants showed a diphasic HR response in association with cutaneous AVA constrictions. The infant response differed from the adult response mainly by the longer duration of the secondary bradycardia. This may reflect different properties of the baroreceptor reflex in infants and adults.

Thermoregulatory fluctuations in heart rate and blood pressure in humans: effect of cooling and parasympathetic blockade.

The spontaneous fluctuations in acral skin blood flow, mean blood pressure (MAP), and heart rate (HR) were studied in 9 healthy supine volunteers in a thermoneutral and in a cool environment, and after parasympathetic blockade by atropine. In skin areas with a high density of arteriovenous anastomoses (AVAs), there were large, spontaneous fluctuations in blood flow in a thermoneutral environment. The fluctuations were nearly abolished in a cool environment, while they seemed unaffected by atropine administration. Power spectral analysis demonstrated a reduction in HR and MAP variability in the low- and mid-frequency (LF) (< 0.15 Hz) band after cooling, and a prominent reduction in HR variability in both the LF and the high-frequency (HF) (< 0.15 Hz) band after atropine administration. A sudden drop in skin vascular conductance was accompanied by diphasic HR changes and an increase in MAP. After atropine administration, the initial HR acceleration was delayed and reduced in magnitude. The secondary HR deceleration, which probably represents baroreceptor modulation, was abolished. Atropine administration caused a delayed, but augmented increase in MAP, which was probably related to the loss of baroreceptor control of HR. The presence of an autonomic rhythm, consisting of sympathetic vasoconstrictor impulses to skin AVAs, connected with reciprocal sympathetic and vagal impulses to the heart, is indicated. The HR changes were mainly determined by vagal activity. The rhythmic changes in skin blood flow, HR and MAP were suppressed in a cool environment.

Fluctuations in blood flow to acral skin in humans: connection with heart rate and blood pressure variability.

1. Spontaneous fluctuations in blood flow in arteries supplying acral skin were investigated with Doppler ultrasound in human subjects. Finger blood pressure, heart rate (HR) and cardiac output were measured simultaneously and noninvasively. 2. Synchronous fluctuations in flow were found in arteries supplying the hands and feet. The fluctuations were larger and more rapid than the flow variations which have been demonstrated with other methods. The magnitude of the total flow fluctuations in the hands and feet was estimated to be 5-10% of cardiac output in resting subjects. This range of flow fluctuations is made possible by spontaneous opening and closing of skin arteriovenous anastomoses (AVAs). 3. The fluctuations in skin blood flow were accompanied by inverse fluctuations in mean blood pressure (MAP). The power spectra of skin vascular conductance and MAP both contained maximum intensity at low frequencies, below 0.15 Hz, with high coherence. 4. The central circulatory events connected with the skin blood flow fluctuations were calculated from the experimental data with the use of transfer function analysis. There was a rise in HR, cardiac output and MAP starting 1-4 s before a cutaneous vasoconstriction. This indicates that the HR and MAP responses are not only passive effects of changes in peripheral resistance, but are the result of a simultaneous activation of the peripheral vascular and cardiac efferent branches of the autonomic nervous system. The HR and MAP responses are then modified, probably by baroreceptor activation.