Effects of RS17053 on α1 -adrenoceptors in rat vas deferens and aorta

BACKGROUND

RS17053 is classed as an α1A -adrenoceptor selective antagonist.

OBJECTIVES

We have examined its profile of action at all subtypes of α1 -adrenoceptor.

METHODS

Noradrenaline (NA) evoked contractions of rat vas deferens involve α1D -adrenoceptors in phasic contractions and α1A -adrenoceptors in tonic contractions. Contractions of rat aorta to NA involve α1D - and α1B -adrenoceptors.

RESULTS

RS17053 (10-5  M) shifted NA potency and virtually abolished tonic contractions to NA, with little or limited effect on phasic contractions. The α1D -adrenoceptor antagonist BMY7378 (3 × 10-7  M) significantly inhibited the remaining phasic component of the contractions, and the α1A -adrenoceptor antagonist RS100329 (10-7  M) inhibited further the residual tonic contraction. Hence, RS17053 shows high selectivity for α1A -adrenoceptors over α1D -adrenoceptors in rat vas deferens. However, RS17053 (10-5  M) produced a large shift in the potency of NA in rat aorta, with a pKB of 6.82. Large shifts of NA potency in rat aorta involve α1B -adrenoceptor blockade.

CONCLUSION

Results in rat vas deferens demonstrate low potency of RS17053 at α1D -adrenoceptors, but results from rat aorta can only be explained as demonstrating α1B -adrenoceptor antagonism by RS17053. RS17053 may be a useful pharmacological tool when reclassified as a mainly α1A - and to a lesser extent α1B -adrenoceptor antagonist with little effect at α1D -adrenoceptors.

Acidosis reduces the function and expression of α1D-adrenoceptor in superior mesenteric artery of Capra hircus

Objective:

The objective of this study was to characterize the α₁-adrenoceptor (α₁-AR) subtypes and evaluate the effect of acidosis on α₁-AR function and expression in goat superior mesenteric artery (GSMA).

Materials and Methods:

GSMA rings were mounted in a thermostatically controlled (37.0°C ± 0.5°C) organ bath containing 20 ml of modified Krebs-Henseleit solution, maintained at pH_o of 7.4, 6.8, 6.0, 5.5, 5.0, and 4.5. Noradrenaline (NA)- and phenylephrine (PE)-induced contractile response was elicited in the absence or presence of endothelium and prazosin at pH_o of 7.4, 6.0, and 5.0. The responses were recorded isometrically by an automatic organ bath connected to PowerLab and analyzed using Labchart 7.1.3 software. Expression of α_1D-AR was compared at physiological and acidic pH_o using reverse transcription-polymerase chain reaction (RT-PCR).

Results:

NA- and PE-induced contractile responses were attenuated proportionately with a decrease in extracellular pH (pH_o), i.e. 7.4 → 6.8 → 6.0 → 5.5 → 5.0 → 4.5. Endothelium denudation increased the contractile response at both normal and acidic pH_o. Prazosin (1 nM, 10 nM, and 0.1 μM) inhibited the NA- and PE-induced contractile response at pH_o 7.4 and the blocking effect of prazosin was potentiated at pH_o of 6.0 and 5.0. RT-PCR analysis for α_1D-AR in GSMA showed that the mRNA expression of α_1D-AR was decreased under acidic pH_o as compared to physiological pH_o.

Conclusion:

(i) Adrenergic receptor mediates vasoconstriction in GSMA under normal physiological pH_o, and α_1D is the possible subtype involved in this event (ii) acidosis attenuates the vasocontractile response due to reduced function and expression of α_1D-AR and also increased the release of endothelial-relaxing factors.

Mechanisms Underlying Activation of α₁-Adrenergic Receptor-Induced Trafficking of AQP5 in Rat Parotid Acinar Cells under Isotonic or Hypotonic Conditions

Defective cellular trafficking of aquaporin-5 (AQP5) to the apical plasma membrane (APM) in salivary glands is associated with the loss of salivary fluid secretion. To examine mechanisms of α₁-adrenoceptor (AR)-induced trafficking of AQP5, immunoconfocal microscopy and Western blot analysis were used to analyze AQP5 localization in parotid tissues stimulated with phenylephrine under different osmolality. Phenylephrine-induced trafficking of AQP5 to the APM and lateral plasma membrane (LPM) was mediated via the α_1A-AR subtype, but not the α_1B- and α_1D-AR subtypes. Phenylephrine-induced trafficking of AQP5 was inhibited by ODQ and KT5823, inhibitors of nitric oxide (NO)-stimulated guanylcyclase (GC) and protein kinase (PK) G, respectively, indicating the involvement of the NO/ soluble (c) GC/PKG signaling pathway. Under isotonic conditions, phenylephrine-induced trafficking was inhibited by La³⁺, implying the participation of store-operated Ca²⁺ channel. Under hypotonic conditions, phenylephrine-induced trafficking of AQP5 to the APM was higher than that under isotonic conditions. Under non-stimulated conditions, hypotonicity-induced trafficking of AQP5 to the APM was inhibited by ruthenium red and La³⁺, suggesting the involvement of extracellular Ca²⁺ entry. Thus, α_1A-AR activation induced the trafficking of AQP5 to the APM and LPM via the Ca²⁺/ cyclic guanosine monophosphate (cGMP)/PKG signaling pathway, which is associated with store-operated Ca²⁺ entry.