Support of arterial blood pressure by major pressor systems in conscious dogs

The effect of intravenous maropitant on blood pressure in healthy awake and anesthetized dogs

OBJECTIVE

To evaluate the effects of intravenous maropitant on arterial blood pressure in healthy dogs while awake and under general anesthesia.

DESIGN

Experimental crossover study.

ANIMALS

Eight healthy adult Beagle dogs.

PROCEDURE

All dogs received maropitant (1 mg kg-1) intravenously under the following conditions: 1) awake with non-invasive blood pressure monitoring (AwNIBP), 2) awake with invasive blood pressure monitoring (AwIBP), 3) premedication with acepromazine (0.005 mg kg-1) and butorphanol (0.2 mg kg-1) intramuscularly followed by propofol induction and isoflurane anesthesia (GaAB), and 4) premedication with dexmedetomidine (0.005 mg kg-1) and butorphanol (0.2 mg kg-1) intramuscularly followed by propofol induction and isoflurane anesthesia (GaDB). Heart rate (HR), systolic (SAP), diastolic (DAP), and mean blood pressures (MAP) were recorded before injection of maropitant (baseline), during the first 60 seconds of injection, during the second 60 seconds of injection, at the completion of injection and every 2 minutes post injection for 18 minutes. The data were compared over time using a Generalized Linear Model with mixed effects and then with simple effect comparison with Bonferroni adjustments (p <0.05).

RESULTS

There were significant decreases from baseline in SAP in the GaAB group (p < 0.01) and in MAP and DAP in the AwIBP and GaAB (p < 0.001) groups during injection. A significant decrease in SAP (p < 0.05), DAP (p < 0.05), and MAP (p < 0.05) occurred at 16 minutes post injection in GaDB group. There was also a significant increase in HR in the AwIBP group (p < 0.01) during injection. Clinically significant hypotension occurred in the GaAB group with a mean MAP at 54 ± 6 mmHg during injection.

CONCLUSION

Intravenous maropitant administration significantly decreases arterial blood pressure during inhalant anesthesia. Patients premedicated with acepromazine prior to isoflurane anesthesia may develop clinically significant hypotension.

Renal vasoconstriction by vasopressin V1a receptors is modulated by nitric oxide, prostanoids, and superoxide but not the ADP ribosyl cyclase CD38

Renal blood flow (RBF) responses to arginine vasopressin (AVP) were tested in anesthetized wild-type (WT) and CD38(-/-) mice that lack the major calcium-mobilizing second messenger cyclic ADP ribose. AVP (3-25 ng) injected intravenously produced dose-dependent decreases in RBF, reaching a maximum of 25 ± 2% below basal RBF in WT and 27 ± 2% in CD38(-/-) mice with 25 ng of AVP. Renal vascular resistance (RVR) increased 75 ± 6% and 78 ± 6% in WT and CD38(-/-) mice. Inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) increased the maximum RVR response to AVP to 308 ± 76% in WT and 388 ± 81% in CD38(-/-) (P < 0.001 for both). Cyclooxygenase inhibition with indomethacin increased the maximum RVR response to 125 ± 15% in WT and 120 ± 14% in CD38(-/-) mice (P < 0.001, <0.05). Superoxide suppression with tempol inhibited the maximum RVR response to AVP by 38% in both strains (P < 0.005) but was ineffective when administered after L-NAME. The rate of RBF recovery (relaxation) after AVP was slowed by L-NAME and indomethacin (P < 0.001, <0.005) but was unchanged by tempol. All vascular responses to AVP were abolished by an AVP V1a receptor antagonist. A V2 receptor agonist or antagonist had no effect on AVP-induced renal vasoconstriction. Taken together, the results indicate that renal vasoconstriction by AVP in the mouse is strongly buffered by vasodilatory actions of NO and prostanoids. The vasoconstriction depends on V1a receptor activation without involvement of CD38 or concomitant vasodilatation by V2 receptors. The role of superoxide is to enhance the contractile response to AVP, most likely by reducing the availability of NO rather than directly stimulating intracellular contraction signaling pathways.

Effects of arginine-vasopressin on regional blood volume distribution in supine humans

In healthy humans, the increase in arterial blood pressure seen in patients with autonomic dysfunction in response to exogenous vasopressin (AVP) is abolished. We tested the hypothesis that redistribution of blood from the intra- to the extrathoracic vascular compartment might contribute to this buffer response. Regional distribution of 99mTc labeled autologous red cells was assessed in healthy supine volunteers (n = 7) during arginine-vasopressin administration (1 ng.kg-1 bolus i.v. followed by a 14-min infusion of 3 ng.kg-1 x min-1), along with arterial and central venous pressures, and heart rate. Exogenous vasopressin increased plasma vasopressin concentration from 4.0 +/- 1.4 SEM to 91 pg.ml-1 +/- 12. Thoracic counts increased slightly but significantly by 2.2% +/- 0.9, while global abdominal counts remained unchanged. Most surprisingly, counts in the liver markedly increased (+8.1% +/- 1.8, p = 0.02), but significantly decreased in the spleen (-3.1% +/- 1.4). Intestinal (-2.5% +/- 2.4) and limb counts did not change significantly. Consistent with the increase in thoracic counts central venous pressure increased from 3.6 mmHg +/- 1 to 4.7 +/- 1 (p = 0.02), while arterial pressure and heart rate did not change. All changes reversed towards baseline when vasopressin administration ceased. Thus, in humans with an intact autonomic system, vasopressin, at concentrations observed during hypotension, increases liver and, albeit to a small extent, also thoracic blood volume, but decreases splenic blood content. These results: 1) are incompatible with the hypothesis that AVP induces a shift of blood from intra- to extrathoracic capacitance vessels, and 2) show that AVP increases rather than decreases central blood volume.