Nitric oxide (NO) is not involved in accentuated antagonism for chronotropy in the isolated mouse atrium.

Nitric oxide (NO) is reportedly involved in accentuated antagonism in the canine blood-perfused sinoatrial (SA) node, and is thought to modulate cholinergic control of heart rate in rabbits. In the present study, we evaluated the involvement of NO in accentuated antagonism in an isolated preparation of both atria from C57BL/6J mice. Isoprenaline (10(-10) M-10(-7) M) had positive chronotropic effects but decreased rather than increased contraction strength. Carbachol (10(-8) M-3x10(-6) M) had a concentration-dependent, negative chronotropic action. In the presence of a submaximal concentration of isoprenaline (30 nM), the same concentration range of carbachol elicited a larger decrease in heart rate than in the absence of isoprenaline. The larger decrease in heart rate (accentuated antagonism) was not modified by the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine methylester (L-NAME). Similar results were obtained using ICR strain mice. Isoprenaline increased cAMP content but not cGMP; carbachol in the presence of isoprenaline increased cGMP but had no further effects on cAMP. In the presence of L-NAME, the increase in cGMP elicited by carbachol was attenuated. In the isolated mouse atria, it appears that NO is not involved in accentuated antagonism due to lack of coupling between cGMP signalling and chronotropic function.

A method for evaluating drug effects on intermittent claudication using a treadmill in rats with unilateral hindlimb artery occlusion.

INTRODUCTION: We have developed an in vivo experimental model for evaluating peripheral arterial insufficiency and predicting the efficacy of drugs on intermittent claudication (IC). We found that rats that had been running normally on a treadmill developed a gait disturbance when a hindlimb artery was unilaterally occluded. We hypothesized that the distance run before gait disturbance developed (DGD) in rats with occlusion of a hindlimb artery might be an appropriate index of the severity of peripheral insufficiency, and that the model might serve as a test bed for evaluating drug efficacy. To prove this hypothesis, we examined whether DGD was determined by severity of hindlimb ischemia. Furthermore, we also examined whether cilostazol, which has been proved to have ameliorative effects in patients with IC, increased DGD. METHODS: To vary the severity of ischemia, either the superficial femoral artery, the distal portion of the iliac artery, or the proximal portion of iliac artery was unilaterally occluded. After a recovery period, these rats were subjected to a treadmill test (15 m/min and 15% incline) to determine DGD and examine the effect of cilostazol on DGD. RESULTS: DGD was the longest and shortest in rats with superficial femoral artery and proximal portion of iliac artery occlusion, respectively. Intermediate DGD was observed in rats with distal portion of iliac artery occlusion. These data suggest that DGD is correlated with the severity of hindlimb ischemia. Two weeks or longer administration of cilostazol 30 and 100 mg/kg twice a day evoked a significant increase in DGD. DISCUSSION: Peripheral arterial insufficiency and its modulation by drugs can be evaluated in rats with unilateral hindlimb artery occlusion, on a treadmill, by measuring DGD.

Effects of OPC-51803, a novel, nonpeptide vasopressin V2-receptor agonist, on micturition frequency in Brattleboro and aged rats.

We assessed the effects of OPC-51803 ((5R)-2-[1-(2-chloro-4-(1-pyrrolidinyl)benzoyl)-2,3,4,5-tetrahydro-1H-1-benzazepin-5-yl]-N-isopropylacetamide), a nonpeptide vasopressin V(2)-receptor agonist, on micturition frequency in female homozygous Brattleboro rats (strain carries hereditary diabetes insipidus) and aged male Sprague-Dawley rats with polyuria. Female homozygous Brattleboro rats exhibited more diuresis and a larger micturition frequency over a 24-h period than did the heterozygous controls. In Brattleboro rats, an oral administration of OPC-51803 at 0.03 and 0.3 mg/kg significantly decreased urinary frequency and was accompanied by decreased urine volume. However, little effect was seen in the mean and maximal micturition volume. Aged male Sprague-Dawley rats (25-month-old) showed a significant increase in urine volume throughout a 0- to 24-h period compared with mature (6-month-old) rats. Orally administered OPC-51803 at 0.3 mg/kg decreased not only urine volume but also urinary frequency in aged rats. Furthermore, OPC-51803 prolonged the time prior to the first micturition. Therefore, OPC-51803 decreased micturition frequency in both rat species by reducing urine outflow. This suggests that the compound will be useful for treating micturition disorders that result in frequent micturition, such as that from polyuria, nocturnal polyuria, and some kinds of urinary incontinence.

Antidiuretic effects of a novel nonpeptide vasopressin V(2)-receptor agonist, OPC-51803, administered orally to dogs.

We elucidated the pharmacological properties of a novel nonpeptide vasopressin V(2)-receptor agonist, OPC-51803 ((5R)-2-[1-(2-chloro-4-(1-pyrrolidinyl)benzoyl-2,3,4,5-tetrahydro-1H-1-benzazepine-5-yl]-N-isopropylacetamide), via both in vitro binding experiments incorporating canine kidney and platelet membrane fractions and in vivo experiments that would determine the compound's antidiuretic effects after oral administration to water-loaded dogs. OPC-51803 displaced [(3)H]arginine vasopressin (AVP) binding to canine V(2) and V(1a) receptors, as determined by resulting K(i) values of 15.2 +/- 0.6 nM (n = 4) and 653 +/- 146 nM (n = 4), respectively. These data indicate that OPC-51803 was about 43 times more selective for V(2) receptors than for V(1a) receptors. Antidiuretic studies showed that orally administered doses of OPC-51803 (0.03 to 0.3 mg x kg(-1)) decreased urine volume and increased urinary osmolality in a dose-dependent manner in water-loaded dogs. Intravenous OPC-51803 infusions (0.3 and 3 microg x kg(-1) x min(-1)) did not affect renal or systemic hemodynamics in anesthetized dogs. Since these results confirm that OPC-51803 shows antidiuretic action in dogs, the compound may be useful for treating AVP-deficient pathophysiological states.