Role of cyclic nucleotides in vasopressin-induced piglet pial artery dilation and opioid release

Effects of adrenaline and vasopressin on cerebral microcirculation at baseline and during global brain ischemia and reperfusion in rabbits

BACKGROUND

During cardiopulmonary resuscitation, the brain becomes ischemic. Adrenaline and vasopressin have been recommended for use during cardiopulmonary resuscitation. We aimed to investigate the direct effects of adrenaline and vasopressin on the cerebral microvasculature at baseline and during ischemia and reperfusion in rabbits.

METHODS

The closed cranial window method was used to visualize the cerebral microcirculation and changes in the pial arteriole diameter in rabbits. Adrenaline and vasopressin were administered topically on the brain tissue. First, the effects of adrenaline and vasopressin on pial arterioles were evaluated in 7 rabbits that were given 4 different concentrations of adrenaline, and another 7 rabbits that received 4 different concentrations of vasopressin. Second, the effects of adrenaline and vasopressin were determined during the global brain ischemia and reperfusion, which was induced by clamping the brachiocephalic, left common carotid, and left subclavian arteries for 15 min. An additional 21 rabbits were randomly assigned to receive artificial cerebrospinal fluid (aCSF) (n = 7), adrenaline 10^-5 mol/L (n = 7), or vasopressin 10^-7 mol/L (n = 7). Each drug was continuously infused from 5 min after the initiation of ischemia until 120 min after reperfusion. The pial arteriole diameters were recorded before and during ischemia, and after reperfusion.

RESULTS

At baseline, adrenaline and vasopressin did not affect the cerebral pial arterioles. During ischemia, vasopressin, but not aCSF and adrenaline constricted the pial vessels. Late in the reperfusion phase, pial diameter became reduced in the vasopressin and aCSF groups whereas pial diameter was higher in the animals treated with adrenaline.

CONCLUSIONS

Adrenaline and vasopressin did not affect pial arterioles at baseline. During reperfusion, adrenaline may counteract the cerebral vasoconstriction.

Clinical implication of the changes of cAMP, TXA2 and PGI2 in CSF of asphyxiated newborns

To evaluate the changes of 3', 5'-cyclic adenosine monophosphate (cAMP), thromboxane A2(TXA2) and prostacyclin (PGI2) in cerebrospinal fluid (CSF) in the asphyxiated newborn and explore their roles in hypoxic-ischemic brain damage (HIBD). Thirty-six full term newborns were divided into 3 groups, including 12 with moderate-severe hypoxic-ischaemic encephalopathy (HIE), 13 with mild HIE, 11 without HIE (control group). The levels of cAMP, TXB2 (TXA2 metabolite) and 6-keto-PGF1 alpha (PGI2 metabolite) in CSF and plasma were measured 36-72 h after birth by RIA, and the concentrations were expressed as nM/L (cAMP), ng/L(TXB2 and 6-keto-PGF1 alpha). The infants were followed-up at 6 and 12 month of age and Mental Development Index (MDI) and Psychomotor Development Index (PDI) were measured using Bayley Scales of Infant Development (BSID). The CSF cAMP level in moderate-severe HIE group was 8.60 +/- 2.40, significantly lower than that of the mild HIE group (14.83 +/- 2.84) and the control group (24.43 +/- 2.39) (for both P < 0.01). The levels of TXB2 and 6-keto-PGF1 alpha in CFS in the moderate-severe HIE group (206.06 +/- 29.74, 168.47 +/- 23.02, respectively) were significantly higher than in the mild HIE group (83.37 +/- 28.57, 131.42 +/- 16.57, respectively, P < 0.01) and the control group (41.77 +/- 21.58, 86.23 +/- 13.05, respectively, P < 0.01). The level changes of cAMP, TXB2 and 6-keto-PGF1 alpha in plasma in all groups were similar to those in CSF, but no significant difference was found between mild HIE group and the control group (P > 0.05). The follow-up results showed that MDI and PDI of the moderate-severe HIE group were the lowest (84.79 +/- 13.34, 83.50 +/- 13.28, respectively), followed by mild HIE group (102.19 +/- 7.02, 99.94 +/- 9.08, respectively), with the control group being the highest (116.63 +/- 12.08, 116.69 +/- 10.87, respectively). Univariate analysis showed some significant difference (the moderate-severe HIE group vs. the mild HIE group or the control group, P < 0.01; the mild HIE group vs. the control group P < 0.05). The results suggested that the concentration of cAMP, TXA2 and T/K ratio in CSF after neonatal asphyxia might be sensitive markers in evaluating the severity of brain damage in early stage and predicting the future outcome.

Skeletal muscle contractions stimulate cGMP formation and attenuate vascular smooth muscle myosin phosphorylation via nitric oxide

Nitric oxide generated by neuronal nitric oxide synthase in contracting skeletal muscle fibers may regulate vascular relaxation via a cGMP-mediated pathway. Neuronal nitric oxide synthase content is greatly reduced in skeletal muscles from mdx mice. cGMP formation increased in contracting extensor digitorum longus muscles in vitro from C57 control, but not mdx mice. The increase in cGMP content was abolished with NG-nitro-L-arginine. Sodium nitroprusside treatment increased cGMP levels in muscles from both C57 and mdx mice. Skeletal muscle contractions also inhibited phenylephrine-induced phosphorylation of smooth muscle myosin regulatory light chain. Arteriolar dilation was attenuated in contracting muscles from mdx but not C57 mice. NO generated in contracting skeletal muscle may contribute to vasodilation in response to exercise.

Role of endothelial nitric oxide synthase in hypoxia-induced pial artery dilation

Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, through the formation of cGMP and the subsequent release of methionine enkephalin and leucine enkephalin in the newborn pig. In separate studies, these opioids also were observed to elicit NO-dependent pial artery dilation, whereas light/dye endothelial injury reduced hypoxic pial dilation. The current study was designed to investigate the role of the endothelial isoform of NO synthase in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. N-iminoethyl-L-ornithine (L-NIO) (10(-6) mol/L), an antagonist that may have greater endothelial NO synthase inhibitory selectivity, had no effect on dilation elicited by hypoxia (PO2 approximately 35 mm Hg) (24 +/- 2 versus 24 +/- 2% in the absence and presence of L-NIO, respectively, n = 8). Hypoxic dilation was accompanied by increased CSF cGMP, which also was unchanged in the presence of L-NIO (394 +/- 19 and 776 +/- 63 versus 323 +/- 13 and 739 +/- 25 fmol/mL for control and hypoxia in the absence and presence of L-NIO, respectively, n = 6). Additionally, hypoxic pial dilation was associated with increased CSF methionine enkephalin, which also was unchanged in the presence of L-NIO (992 +/- 73 and 2469 +/- 197 versus 984 +/- 18 and 2275 +/- 185 pg/mL, respectively, n = 6). In contrast, methionine enkephalin-induced dilation was blocked by L-NIO (6 +/- 1, 10 +/- 1, and 16 +/- 1 versus 1 +/- 1, 1 +/- 1, and 2 +/- 1% for 10(-10), 10(-8), 10(-6) mol/L methionine enkephalin, respectively, before and after L-NIO, n = 8). Substance P-induced pial dilation was blunted by L-NIO, whereas responses to sodium nitroprusside and N-methyl-D-aspartate were unchanged. These data indicate that endothelial NO synthase contributes to opioid-induced pial artery dilation but not hypoxia-induced dilation. Additionally, these data suggest that neuronally derived NO contributes to hypoxic pial dilation.