William Bloomquist

Aryl propanolamines : comparison of activity at human β3 receptors, rat β3 receptors and rat atrial receptors mediating tachycardia

The in vitro activity of four aryl propanolamines was compared to two prototypic β3 receptor agonists, CGP 12177 and CL316243 at the human β3 receptor, the rat β3 receptor in the stomach fundus and receptors mediating atrial tachycardia. L‐739,574 was the most potent (EC50=9 nM) and selective agonist at the human β3 receptor with high maximal response (74% of the maximal response to isoproterenol). A phenol‐biaryl ether analogue possessed modest affinity for the human β3 receptor (EC50=246 nM), but was highly efficacious with a maximal response 82% of the maximal response to isoproterenol. The other derivatives were intermediate in potency with low maximal responses. These agonists at the human β3 receptor did not activate the rat β3 receptor in the rat stomach fundus. In fact, the aryl propanolamines (10−6 M) inhibited CL316243‐induced activation of the rat β3 receptor. Thus, agonist activity at the human β3 receptor translated into antagonist activity at the rat β3 receptor. L739,574 and the phenol biaryl ether increased heart rate via β1 receptors. Although CGP12177 produced atrial tachycardia, neither the indole sulphonamide nor biphenyl biaryl ether did, although both had high affinity for the human β3 receptor. Thus, the atrial tachycardic receptor was not identical to the human β3 receptor. These studies (a) characterized four aryl propanolamines with high affinity at the human β3 receptor, (b) found that they were antagonists at the rat β3 receptor, an observation with profound implications for in vivo rat data, and (c) established that the rodent atrial non‐β1, β2 or β3 tachycardic receptor was also unrelated to the human β3 receptor.

MCPP but not TFMPP is an antagonist at cardiac 5HT3 receptors

The prototypic arylpiperazines, meta-chlorophenylpiperazine (mCPP), meta-trifluoromethylphenylpiperazine (TFMPP) and quipazine are widely studied serotonergic ligands with nonselective effects at 5HT1 and 5HT2 receptor subtypes. The present study was designed to compare the affinities of these arylipiperazines at 5HT3 receptors, and to determine agonist or antagonist activity at 5HT3 receptors. Quipazine showed high affinity at brain 5HT3 receptors (IC50 = 4.4 nM) and was a potent agonist of the von Bezold-Jarisch reflex in anesthetized rats, a response mediated by cardiac 5HT3 receptors. In concentrations that activated 5HT3 receptors, quipazine also antagonized serotonin-induced bradycardia in anesthetized rats. Taken together, these data suggest that quipazine is an agonist/antagonist with high affinity at 5HT3 receptors in both brain and cardiac tissue. Although mCPP also showed relatively high affinity at brain 5HT3 receptors (IC50 = 61.4 nM), it did not activate the von Bezold-Jarisch reflex; instead, mCPP potently antagonized serotonin-induced bradycardia. Thus, mCPP acts as an antagonist at 5HT3 receptors in the periphery. Although both quipazine and mCPP possessed relatively high affinity at brain 5HT3 receptors, TFMPP did not bind appreciably to 5HT3 receptors in brain (IC50 = 2373 nM) and neither activated nor inhibited cardiac 5HT3 receptors. That TFMPP did not interact with 5HT3 receptors, whereas quipazine and mCPP did, is in marked contrast to the similar effects of all three arylpiperazines at other serotonin receptors. The selectivity of TFMPP for 5HT1 and 5HT2 receptors (i.e., its minimal affinity for 5HT3 receptors) suggests that this arylpiperazine may be a preferred ligand relative to mCPP when studying 5HT1 or 5HT2 receptor mediated responses.

5-HT4 receptors in rat but not guinea pig, rabbit or dog esophageal smooth muscle.

1. Marked heterogeneity among species exists in the esophageal response to pharmacological agents. The present study compared the response to serotonin in esophagus from the rat, guinea pig, rabbit and dog. 2. The esophagus from all four species contracted to carbamylcholine and to PGF2 alpha; responses to serotonin were the most variable among species. 3. Serotonin contracted the guinea pig and rabbit esophagus; an effect blocked by LY53857 (10(-7 M) and ketanserin (10(-7) M), consistent with 5-HT2 receptor activation mediating this contraction. 4. Serotonin neither contracted nor relaxed the canine esophagus and relaxed the rat esophagus via 5-HT4 receptor activation as determined by antagonism with ICS 205-930 (-log KB = 6.4), metoclopramide (-log KB = 6.7) and its ester congener SDZ 205-557 (-log KB = 7.9). Two methylene homologs of SDZ 205-557 also had high 5-HT4 receptor affinity (-log KB = 7.7). 5. Thus, in guinea pig and rabbit esophagus, serotonin induced a contraction mediated by 5-HT2 receptors; and serotonin neither contracted nor relaxed the canine esophagus. In rat esophagus, serotonin induced a relaxation mediated by activation of 5-HT4 receptors.

Effect of acute and subchronic subcutaneous urocortin on blood pressure and food consumption in ob/ob mice.

Corticotropin-releasing factor and urocortin belong to a hypothalamic peptide family thought to be important in appetite regulation. The present study compared the appetite-suppressant effect of subcutaneous urocortin in obese mice to its cardiovascular effects. Acutely, urocortin (100 nmol/kg iv) reduced blood pressure and increased heart rate in urethane anesthetized nonobese mice; effects similar to those produced by subcutaneous urocortin (10 and 100 nmol/kg sc) in nonobese and ob/ob mice. Over this same dose range (10-100 nmol/kg sc), urocortin dramatically inhibited food consumption in the ob/ob mouse. To determine if the acute hypotensive effect of urocortin (10 nmol/kg sc) in the ob/ob mouse persisted after repeated urocortin administration, animals were pretreated for 3 days with urocortin (10 nmol/kg sc) or vehicle. Following urocortin pretreatment, urocortin-induced hypotension was similar to the effect in vehicle pretreated mice. However, urocortin-induced appetite suppression was reduced following 3 days of pretreatment with urocortin (10 nmol/kg sc) to ob/ob mice. These data suggest that the hypotensive and appetite-suppressant effects of urocortin are mediated by different mechanisms and tolerance to the hypotension did not readily occur in obese animals.

Lack of a difference between ketanserin and ritanserin in central vs. peripheral serotonin receptor antagonism

Ketanserin and ritanserin antagonized with similar potency the pressor response to serotonin in pithed rats, a measure of antagonism of vascular 5HT2 receptors. Both compounds also antagonized the elevation of serum corticosterone concentration by quipazine, a centrally acting serotonin agonist; higher doses of both antagonists were needed, but ketanserin was not less potent than ritanserin. Earlier suggestions that ketanserin mainly blocks peripheral serotonin receptors and that ritanserin mainly blocks central serotonin receptors seem unfounded.

Comparative 5‐HT4 receptor antagonist activity of LY353433 and its active hydroxylated metabolites

LY353433 is a selective, potent, and orally active 5‐HT4 receptor antagonist with a long duration of pharmacological activity following its oral administration to rats. After oral administration of LY353433 (100 mg/kg) to rats, peak plasma concentration of the parent material was approximately 25 ng/ml and rapidly declined such that 4 h after administration, plasma concentration of the parent material was barely detectable. However, two additional peaks (LY343031 and LY343032) were observed in plasma via HPLC and subsequently identified as hydroxylated metabolites of LY353433. Peak plasma concentrations of LY343031 (approximately 50 ng/ml) and of LY343032 (approximately 150 ng/ml) were achieved within 1 h after the oral administration of LY353433. Furthermore, plasma concentrations of these metabolites were maintained for several hours and declined more slowly than plasma concentrations of the parent material. Both metabolites inhibited esophageal 5‐HT4 receptors in a concentration range similar to that observed with LY353433. In addition, the receptor selectivity profiles for LY343031 and LY343032 were similar to that of LY353433 at α1, α2, β, Dopamine D1, Dopamine D2, benzodiazapine, histamine H1, GABAA, 5‐HT2, and muscarinic receptors. Thus, these hydroxylated derivatives of LY353433 were potent and selective 5‐HT4 receptor antagonists in vitro. Lastly, using ex vivo esophageal relaxation to serotonin to assess 5‐HT4 receptor antagonism, these compounds were less active after oral administration to rats than LY353433. Thus, the long duration of pharmacological activity observed after oral administration of LY353433 is likely related not only to the 5‐HT4 receptor antagonist activity of the parent molecule, but also to the 5‐HT4 receptor antagonist activity associated with its two hydroxylated metabolites. Drug Dev. Res. 43:193–199, 1998.