Renal blood flow and its distribution following haemorrhage in the rat: the role of vasopressin

The 5-hydroxytryptamine1A receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin, increases cardiac output and renal perfusion in rats subjected to hypovolemic shock

The 5-hydroxytryptamine(1A) receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), raises blood pressure (BP) and venous tone in rats subjected to hemorrhagic shock. Here, BP, ascending aortic blood flow [i.e., estimate of cardiac output (CO)] and venous blood gases were measured to determine the hemodynamic effects of 8-OH-DPAT (30 nmol/kg i.v., n = 10), saline (n = 10), or an equipressor infusion of epinephrine (n = 10) in unanesthetized rats subjected to hemorrhagic shock (25 min of hypotensive hemorrhage, approximately 50 mm Hg). Renal and iliac blood flow were measured in separate groups of similarly hemorrhaged rats given the same dose of 8-OH-DPAT (n = 7) or saline (n = 6). Compared with saline treatment, 8-OH-DPAT produced a sustained rise in BP (+32 +/- 4 versus +9 +/- 2 mm Hg, 15 min after injection, P < 0.01) and CO (+27 +/- 5 versus +4 +/- 6 ml/min/kg, P < 0.01) but did not affect total peripheral resistance (TPR). Infusion of epinephrine reduced CO (-12 +/- 6 ml/min/kg, P < 0.01) and dramatically increased TPR [+0.37 +/- 0.11 versus +0.05 +/- 0.05 log (mm Hg/ml/min/kg), P < 0.01]. 8-OH-DPAT increased renal conductance (+7 +/- 1 versus +4 +/- 1 microl/min/mm Hg, P < 0.01) but did not significantly affect iliac conductance. 8-OH-DPAT attenuated further development of acidosis compared with either saline or epinephrine (-5.6 +/- 1.6 versus -13.0 +/- 2.0 versus -11.3 +/- 2.6 mmol/liter base excess 45 min after start of hemorrhage, both P < 0.01 versus 8-OH-DPAT). These data demonstrate that 8-OH-DPAT improves hemodynamics during circulatory shock, in part, through renal vasodilation and mobilizing of blood stores.

The renal vascular response to mild and severe haemorrhage in the anaesthetized rat

1. In order to document the characteristics of the renal vascular response to blood losses of varying severity, Inactin-anaesthetized rats were subjected to a haemorrhage of 5, 10, 15 or 20 ml (kg body weight)-1, while a fifth group (control rats) remained unbled. Radioactive microspheres (diameter 10 microns) were used to determine cardiac output, total renal blood flow and the distribution of blood flow within the kidneys; measurements were made before and 5-120 min after haemorrhage. 2. In control animals none of the variables studied changed significantly during the experimental period. 3. Immediately after haemorrhage there were reductions in arterial pressure and cardiac output which were roughly proportional to the severity of haemorrhage. Arterial pressure recovered to some extent during the next 30 min, then stabilized; cardiac output recovered only slightly. 4. Total renal blood flow fell to an extent dependent on the degree of haemorrhage, with no evidence of subsequent recovery. The approximate reductions in renal blood flow were 2% (n.s.), 15%, 30% and 50% after bleeds of 5, 10, 15 and 20 ml kg-1, respectively. Renal vascular resistance increased consistently only in the groups bled 15 and 20 ml kg-1. When renal blood flow was expressed as a fraction of cardiac output, it increased during the period immediately after haemorrhage, indicating some degree of 'protection' of the renal circulation in the face of hypotension. 5. Measurements of intrarenal blood flow indicated a significantly reduced flow to the superficial cortex after every degree of haemorrhage. Inner cortical flow was less affected and fell significantly only in the groups bled 15 and 20 ml kg-1; blood flow to the mid-cortex was intermediate.

Influence of renal denervation on vascular responsiveness of isolated rat intrarenal arteries

Microsurgical renal denervation of the rat has been reported to increase blood loss and bleeding time after a standardized kidney resection. To investigate the vascular effects of denervation, isolated intrarenal arteries were studied using sensitive 'isometric' recording equipment. Four pieces of evidence were obtained to indicate an effective functional denervation I week after renal nerve transection: (i) Phentolamine reduced the K+-induced contraction in controls but not in denervated arteries. (ii) The K+-induced contraction was significantly smaller in denervated than in control arteries. (iii) Noradrenaline (NA) was a significantly more potent vasoconstrictor (4 x) in denervated than in control arteries. (iv) Cocaine increased the NA sensitivity in control arteries (3 x), whereas it failed to do so in denervated vessels. Vasopressin, 5-hydroxytryptamine (5-HT), NA (in the presence of cocaine), prostaglandin F2 alpha (PGF2 alpha) and dopamine (DA) produced concentration-dependent contractions in the mentioned order of potency. Denervated arteries were found to be about two to three times more sensitive to the vasoconstrictors than control arteries. Angiotensin I and II had no contractile effect in any of the vessel segments examined. Indomethacin-pretreated arteries also failed to respond to angiotensin II. Neuropeptide Y produced only weak contractions and failed to influence the NA concentration-response relationship in either control or denervated arteries. In conclusion, renal denervation caused a general supersensitivity of the vascular smooth muscle cells to both circulating and non-circulating vasoconstrictors. Our results cannot explain the increased blood loss and bleeding time seen after denervation, but rather support the view that the enhanced bleeding was caused by an interrupted vasoconstrictor influence of the sympathetic nerves.

The cardiovascular effects of vasopressin after haemorrhage in anaesthetized rats

The cardiovascular effects of an acute haemorrhage (2% of the body weight) were studied over a 60 min period in three groups of rats: (a) Brattleboro rats with hereditary hypothalamic diabetes insipidus (b.d.i.) lacking circulating vasopressin, (b) control rats of the parent Long Evans (l.e.) strain, and (c) l.e. rats treated with an antagonist of the vascular action of vasopressin. Prior to the haemorrhage there were no significant differences between the three groups of rats with respect to mean arterial blood pressure, cardiac output, stroke volume or total peripheral resistance. Following the haemorrhage cardiac output and stroke volume were severely reduced in all three groups of rats. Total peripheral resistance was relatively unaffected in antagonist-treated l.e. rats and b.d.i. rats, but rose substantially in response to the loss of blood in the control l.e. group. Both total peripheral resistance and mean arterial blood pressure were markedly greater in the untreated l.e. control rats than in the other two groups of animals during the first 20 min after haemorrhage. The mean heart rate measured in Brattleboro rats was elevated compared with that of control l.e. rats throughout the experiment and, in addition, significantly greater than that of antagonist-treated l.e. rats during the first 40 min after the haemorrhage. Survival rate for the b.d.i. rats following the 2% haemorrhage was lower than that for l.e. control rats and antagonist-treated l.e. rats. The results indicate that the recovery of the blood pressure following an acute arterial haemorrhage is significantly influenced by vasopressin, particularly during the first 20 min, and that the predominant effect of the hormone is to increase the total peripheral resistance. The higher mortality associated with volume depletion in the b.d.i. rats is unlikely to be directly related to the absence of the vascular action of vasopressin, since administration of the vasopressin antagonist to normal l.e. rats does not reduce their survival rate.