The effects of beta-adrenoceptor agonists on KCl-induced rhythmic contraction in the ureter of guinea pig

β-Adrenergic Receptor Subtypes in the Urinary Tract

Within the urinary tract, β-adrenergic receptors (AR) are found largely on smooth muscle cells but are also present, at least in the bladder, in the urothelium and on afferent nerves. Our understanding of β-AR subtype expression and function is hampered by a lack of well-validated tools, particularly with regard to β(3)-AR. Moreover, the β-AR subtypes involved in a specific function may differ between species. In the ureter, β-AR can modulate pacemaker activity and smooth muscle tone involving multiple subtypes. In the human bladder, β-AR promote urine storage. Bladder smooth muscle relaxation primarily involves β(3)-AR, and the agonists selective for this subtype are in clinical development to treat bladder dysfunction. While prostate and urethra also express β-AR, the overall physiological role in these tissues remains unclear.

Roles of PKA, PI3K, and cPLA2 in the NO-mediated negative inotropic effect of β2-adrenoceptor agonists in guinea pig right papillary muscles

Although β2-adrenoceptors represent 15–25% of β-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of β2-adrenoceptor partial agonists in right papillary muscles. Salbutamol induced a small but significant concentration-dependent negative inotropic effect (NIE, −5% at 60 nM) followed by a moderate positive inotropic effect (+36% at 6 μM) due to activation of β1-adrenoceptors. In the presence of 4 μM atenolol, the concentration-dependent NIE (−12% at 6 μM) was biphasic, best described by a double logistic equation with respective EC50 values of 3 and ∼420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/cytosolic phospholipase A2 (cPLA2) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by cPLA2. The possibility that these effects are due to an equilibrium between different affinity states of the receptor (Gs/Gi coupled and Gi independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that β2-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state.

Demonstration of intrinsic innervation of the guinea pig upper urinary tract using whole-mount preparation

AIMS

The morphology and functional importance of the autonomic nervous system in the upper urinary tract is still not completely understood. Previous histological studies investigating the innervation of the urinary tract have mainly used conventional sections in which the three-dimensional structure of the intramural innervation is difficult to achieve. In contrast, the whole-mount preparation technique is a suitable method for visualizing the distribution of the mesh-like neuronal networks within the urinary tract.

METHODS

The distribution and regional variation of neurofilament (NF), tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), and substance P-immunoreactive (SP-IR) neurons, as well as acetylcholinesterase (AChE) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive neurons were investigated using whole-mount preparations of the guinea pig upper urinary tract.

RESULTS

Two distinct nervous plexuses were detected within the muscle layers containing NF, TH, ChAT, and SP-IR nerves. AChE-positive nerves were seen in all layers. Only moderate NADPH-d-positive innervation was found. Renal pelvis, upper and lower part of the ureter showed an overall increased innervation compared to the middle portion of the ureter. Ganglia were found at the pelviureteric border displaying NF and TH immunoreactivity.

CONCLUSION

The whole-mount preparation technique provides an elegant method for assessing the three-dimensional architecture of ureteral innervation. The guinea pig upper urinary tract is richly supplied with adrenergic, cholinergic, nitrergic, and sensory nerves which suggest that the autonomous nervous system plays an important role in controlling ureteral motility and blood flow.

New insights into beta-adrenoceptors in smooth muscle: distribution of receptor subtypes and molecular mechanisms triggering muscle relaxation

1. The beta-adrenoceptor is currently classified into beta(1), beta(2) and beta(3) subtypes and all three subtypes are expressed in smooth muscle. Each beta-adrenoceptor subtype exhibits tissue-specific distribution patterns, which may be a determinant controlling the mechanical functions of corresponding smooth muscle. Airway and uterine smooth muscles abundantly express the beta(2)-adrenoceptor, the physiological significance of which is established as a fundamental regulator of the mechanical activities of these muscles. Recent pharmacomechanical and molecular approaches have revealed roles for the beta(3)-adrenoceptor in the gastrointestinal tract and urinary bladder smooth muscle. 2. The beta-adrenoceptor is a G(s)-protein-coupled receptor and its activation elevates smooth muscle cAMP. A substantial role for a cAMP-dependent mechanism(s) is generally believed to be the key trigger for eliciting beta-adrenoceptor-mediated relaxation of smooth muscle. Downstream effectors activated via a cAMP-dependent mechanism(s) include plasma membrane K(+) channels, such as the large-conductance, Ca(2+)-activated K(+) (MaxiK) channel. 3. Beta-Adrenoceptor-mediated relaxant mechanisms also include cAMP-independent signalling pathways. This view is supported by numerous pharmacological and electrophysiological lines of evidence. In airway smooth muscle, direct activation of the MaxiK channel by G(s)alpha is a mechanism by which stimulation of beta(2)-adrenoceptors elicits muscle relaxation independently of the elevation of cAMP. 4. The cAMP-independent mechanism(s) is also substantial in beta(3)-adrenoceptor-mediated relaxation of gastrointestinal tract smooth muscle. However, in the case of the beta(3)-adrenoceptor, a delayed rectified K(+) channel rather than the MaxiK channel seems to mediate, in part, cAMP-independent relaxant mechanisms. 5. In the present article, we review the distribution of beta-adrenoceptor subtypes in smooth muscle tissues and discuss the molecular mechanisms by which each subtype elicits muscle relaxation, focusing on the roles of cAMP and plasma membrane K(+) channels.

Goat ureter-an alternative model for measuring ureteral peristalsis

An alternative model for the measurement of ureteral peristalsis is described using the goat ureter. Ureters from freshly slaughtered goats (Capra aegagous hircus) were collected from a local slaughter house. The peristaltic reflex of these preparations was recorded using a specially designed apparatus. The preparations were mounted so that contractile responses to drugs could be recorded isometrically. Histological studies were undertaken to enable a correlation to be made between the anatomical observations and the functional studies. The spontaneous peristaltic reflex of the goat ureter (7 +/- 2 per 2 min) showed a 50% increase in the frequency of contraction (13.66 +/- 1.6, P<0.001) after application of histamine at a concentration of between 6.512 microM and 13.024 microM, but was blocked completely by 10.4 microM of pheniramine (P>0.05). The reflex was not blocked by the H2 blocker ranitidine (P<0.001). The effects of acetylcholine were variable. Calcium chloride at 6.8 microM resulted in a tetanic response (P<0.001). Nicorandil showed partial inhibition of spontaneous peristaltic reflex at 189.4 microM and complete inhibition at 473.4 microM (P<0.001). Although acetylcholine did not show any appreciable effect on the isometric contractions at a maximum dose of 275.2 microM, adrenaline increased the frequency of contractions by 8.2 +/- 6.5 (P<0.001), while salbutamol and isoprenaline had no effect. The histology revealed a striking resemblance to the human ureter, with a structure that explained the responses obtained. The anatomic, physiologic and histological similarities to the human ureter make it an effective alternative in tropical countries for research on ureteral peristalsis.