This study compared the contractile responses elicited by angiotensin II (AII), arginine vasopressin (AVP), and 5-hydroxytryptamine (5-HT) in isolated occipital arteries (OAs) from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. OAs were isolated and bisected into proximal segments (closer to the common carotid artery) and distal segments (closer to the nodose ganglion) and mounted separately on myographs. On day 9, 2K-1C rats had higher mean arterial blood pressures, heart rates, and plasma renin concentrations than SHAM rats. The contractile responses to AII were markedly diminished in both proximal and distal segments of OAs from 2K-1C rats as compared to those from SHAM rats. The responses elicited by AVP were substantially greater in distal than in proximal segments of OAs from SHAM rats and that AVP elicited similar responses in OA segments from 2K-1C rats. The responses elicited by 5-HT were similar in proximal and distal segments from SHAM and 2K-1C rats. These results demonstrate that continued exposure to circulating AII and AVP in 2K-1C rats reduces the contractile efficacy of AII but not AVP or 5-HT. The diminished responsiveness to AII may alter the physiological status of OAs
The cell bodies of vagal afferents are located in the nodose ganglia [
The likelihood that OAs respond to circulating factors is supported by evidence that 5-HT contracts OAs harvested from anesthetized patients with brain tumors and/or aneurysms via activation of
To date, no studies have examined the contractile properties of OAs upon development of 2-kidney, 1-clip (2K-1C) induced hypertension, an established animal model of human renovascular hypertension [
This investigation conforms to the Guide for the Care and Use of Laboratory Animals from the National Institutes of Health (NIH Publication number 85-23, revised 1996). All protocols were approved by the University of Georgia Animal Care and Use Committee.
Male Sprague-Dawley rats (380–400 g) were assigned to two experimental study groups (
Nine days after sham- or renal-clipping, the sham-operated (SHAM) and 2K-1C rats were weighed and anesthetized with 2% isoflurane delivered in 95% O2 and 5% CO2. A polyethylene catheter (PE-3, Scientific Commodities Inc., Lake Havasu City, AZ) was advanced inside the left femoral artery. Mean and peak systolic arterial blood pressures were measured via a pressure transducer (Radnoti LLC, Monrovia, CA) connected to a data acquisition system (LabChart, AD Instruments, Colorado Springs, CO). Blood was collected in EDTA (7.2 mg, Vacutainer, BD, Franklin Lakes, NJ) via the arterial catheter. Plasma renin concentrations were measured in triplicate via a fluorometric kit (SensoLyte 520 Rat Renin Assay Kit, AnaSpec, Inc., Fremont, CA), as described previously [
Following blood collection, the rats were killed by decapitation under isoflurane anesthesia. Heart weights and the weights of both kidneys were recorded. In addition, heart weight/body weight and kidney weight/body weight ratios were also determined.
Following euthanasia, the rat heads were immediately placed in ice cold physiological saline solution (PSS) containing (mM) NaCl 118, NaHCO3 24, KCl 4, glucose 5.6, MgSO4 1, NaH2PO4 0.435, and CaCl2 1.8. OAs (250–400
[Arg8]-Vasopressin acetate salt and 5-hydroxytryptamine hydrochloride were from Sigma-Aldrich (St. Louis, MO). Angiotensin II was from American Peptide Co. (Sunnyvale, CA).
All data are presented as mean ± SEM. Contractile responses elicited by the vasoconstrictors were expressed as a percentage of the maximal contractile response elicited by KPSS (
Data relevant to the SHAM and 2K-1C rats as measured 9 days after surgery are summarized in Table
Body and tissue weight parameters in SHAM and 2K-1C rats on day 9.
Parameter | SHAM | 2K-1C |
---|---|---|
Mean arterial blood pressure, mmHg | 96 ± 1 | 120 ± 4* |
Peak systolic arterial blood pressure, mmHg | 105 ± 3 | 134 ± 5* |
Heart rate, beats/min | 340 ± 12 | 396 ± 9* |
Plasma renin concentration, |
68 ± 2 | 106 ± 13* |
Body weight, g | 426 ± 13 | 413 ± 14 |
Nonclipped kidney weight, g | 1.2 ± 0.04 | 1.4 ± 0.06* |
Nonclipped kidney weight/body weight, mg/g | 2.9 ± 0.08 | 3.3 ± 0.09* |
Clipped kidney weight, g | 1.2 ± 0.05 | 1.0 ± 0.03* |
Clipped kidney weight/body weight, mg/g | 2.8 ± 0.08 | 2.4 ± 0.09* |
Heart weight, g | 1.35 ± 0.04 | 1.48 ± 0.06* |
Heart weight/body weight, mg/g | 3.2 ± 0.04 | 3.6 ± 0.08* |
The data are presented as mean ± SEM. SHAM: sham-operated rats; 2K-1C: 2-kidney, 1-clip rats. There were 8 rats in each group.
The contractile responses elicited by AII in proximal and distal OA segments from SHAM and 2K-1C rats are summarized in Figure
Contractile effects of agonists on proximal and distal segments of the occipital artery.
Agonist | Parameter | Proximal segments | Distal segments | ||
---|---|---|---|---|---|
SHAM | 2K-1C | SHAM | 2K-1C | ||
Angiotensin II | EC50, nM | 6.5 ± 1.3 | 6.8 ± 1.5 | 4.1 ± 1.4 | 3.0 ± 1.6 |
|
66.8 ± 10.1 | 18.5 ± 3.8* | 44.9 ± 7.8 | 14.0 ± 2.9* | |
AVP | EC50, nM | 2.5 ± 1.3 | 2.2 ± 1.5 | 1.5 ± 1.5 | 1.2 ± 1.5 |
|
28.8 ± 4.0 | 25.7 ± 5.0 | 79.2 ± 7.3† | 72.3 ± 8.6† | |
5-HT | EC50, nM | 1,941 ± 119 | 2,852 ± 120* | 844 ± 110† | 941 ± 113† |
|
113.3 ± 6.5 | 100.3 ± 6.2 | 121.3 ± 2.8 | 112.2 ± 6.2 |
Data are presented as mean ± SEM. AVP: arginine vasopressin. 5-HT: 5-hydroxytryptamine. EC50: concentration causing 50% maximal contraction.
Dose-response effects of angiotensin II on tension of proximal and distal occipital artery segments from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. The data are presented as mean ± SEM of the responses expressed as a percentage of that elicited by 80 mM K+ (%
AVP elicited concentration-dependent contractions in proximal and distal OA segments taken from SHAM and 2K-1C rats (Figure
Dose-response effects of arginine vasopressin on tension of proximal and distal occipital artery segments from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. The data are presented as mean ± SEM of the responses expressed as a percentage of that elicited by 80 mM K+ (%
5-HT elicited concentration-dependent contractions in OA segments from SHAM and 2K-1C rats (Figure
Dose-response effects of 5-hydroxytryptamine on tension of proximal and distal occipital artery segments from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. The data are presented as mean ± SEM of the responses expressed as a percentage of that elicited by 80 mM K+ (%
AII and 5-HT elicited robust responses in CCAs from SHAM rats whereas AVP elicited minor effects (Figure
Contractile effects of angiotensin II, AVP, and 5-HT receptor agonists in common carotid arteries from 2K-1C and SHAM rats.
Constrictor | EC50, nM |
|
||
---|---|---|---|---|
SHAM | 2K-1C | SHAM | 2K-1C | |
Angiotensin II | 7.9 ± 1.2 | 7.0 ± 1.2 | 73.6 ± 18.9 | 13.1 ± 3.9* |
AVP | 2.8 ± 1.5 | 10.2 ± 1.9* | 7.4 ± 3.5 | 3.5 ± 1.1 |
5-HT | 3,246 ± 144 | 2,204 ± 121* | 98.8 ± 26.7 | 121.0 ± 21.4 |
Data are presented as mean ± SEM. AVP: arginine vasopressin. 5-HT: 5-hydroxytryptamine. EC50: concentration causing 50% maximal contraction.
Dose-response effects of angiotensin II (a), arginine vasopressin (b), and 5-hydroxytryptamine (c) on tension of common carotid arteries from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. The data are presented as mean ± SEM of the responses expressed as a percentage of that elicited by 80 mM K+ (%
With respect to arteries from SHAM rats, (1) EC50 and
This study employed our novel renal artery clips to elicit 2K-1C hypertension in rats [
A major finding of this study was that AII elicited markedly smaller increases in tension in CCAs and proximal and distal OA segments from 2K-1C rats than from SHAM rats. We have reported that AII and the selective AT1 receptor agonist, val5-angiotensin II, contracted proximal and distal segments of OAs taken from conscious Sprague-Dawley rats, whereas no evidence was found for the presence of AT2 receptors in these OA segments [
The AII-mediated contractions in the OA segments were greater than those observed in our earlier study [
AVP elicited robust increases in tension in distal OA segments from SHAM rats and substantially smaller contractions in proximal OA segments from these rats. AVP elicited even smaller responses in CCAs than in proximal OAs. The relative potencies of AVP in the proximal and distal OA segments were very similar to those reported in our earlier study [
An important observation of the present study was that the contractile effects of AVP in both proximal and distal OA segments from 2K-1C rats were virtually identical to those of the corresponding segments from SHAM rats. It therefore appears that the mechanisms that elicit desensitization and/or downregulation of AII AT1 receptors in 2K-1C hypertensive rats do not affect AVP V1 receptors. However, it should be noted that the direct contractile effects of AVP are subject to tachyphylaxis in rat aortic rings [
5-HT and the selective 5-HT2 receptor agonist,
An important observation of the present study was that the contractile effects of 5-HT in CCAs and proximal and distal segments of OAs from 2K-1C rats were similar to those of the corresponding segments from SHAM rats. As such, the mechanisms that elicit desensitization and/or downregulation of AII AT1 receptors in 2K-1C hypertensive rats do not affect 5-HT2 receptors. It should be noted that the direct contractile effects of 5-HT on arteries are subject to tachyphylaxis via mechanisms in the smooth muscle [
In summary, the potency and maximal effects of AII were similar in the CCA and proximal and distal segments of the OA. In contrast, AVP elicited minor responses in the CCA, slightly more pronounced responses in the proximal OA (that closest to the CCA), and pronounced responses in the distal OA (that closest to the nodose ganglion). The graded increase in the maximum responses to AVP from the CCA to the distal OA in SHAM and in 2K-1C rats (see Tables
Our finding that the contractile effects of AII on OAs of 2K-1C rats were diminished whereas the contractile effects of AVP and 5-HT were not has direct implications for the status of blood flow to nodose ganglia and the delivery of blood-borne factors to vagal afferent cells bodies within these ganglia
AII [
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was funded by the National Institutes of Health (1RO1NS054117-01A2).