Irreversible agonist and antagonist properties of isothiocyanatobenzyl imidazoline in albino rabbit iris muscles

Abstract

The alkylating agent isothiocyanatobenzyl imidazoline (IBI) was synthesized to investigate the unique receptor interacting properties of imidazolines. On the isolated rabbit iris sphincter, IBI produced concentration-dependent responses with an EC50 of 18 mumol/1, and at the highest concentration tested the maximum contraction of the tissue was 50% of the carbachol maximum. At equiactive concentrations with the similar washing procedure, the total duration of responses to IBI and carbachol was 24 and 3 min, respectively. After repeated washing, the sphineter relaxes to the control baseline of tone but, after reexposure to IBI for 6 h, failed to contract, indicating that desensitization or irreversible block has developed. Unlike with carbachol, the sphincter contraction to IBI was not affected by atropine 1 mumol/1, indomethacin 1 mumol/1, verapamil 10 mumol/1, or nifedipine 10 mumol/1. At a higher concentration of nifedipine and papaverine 100 mumol/1, the response to IBI was blocked. Furthermore, the contractile response to IBI was abolished by Ca++ removal from the medium. Under similar conditions, 26 +/- 8% of the maximum response to carbachol was preserved. Thus influx of extracellular as well as rise in intracellular Ca++ appears vital for the contractile response to IBI. IBI did not contract the iris dilator, but shifted the concentration-response curve to the alpha-adrenoceptor activator, phenylephrine, to the right with a reduction in the maximum response. The tissue failed to regain the sensitivity to phenylephrine after 6 h of repeated washing. Phentolamine and nifedipine provided a small but significant protection of the response to phenylephrine against the irreversible block by IBI. Based on chemical and pharmacological properties of IBI, it is concluded that the molecule acts in the rabbit as an irreversible agonist on unidentified receptors of the iris sphineter and an irreversible antagonist of multiple receptors on the iris dilator. These molecular properties of IBI are clearly different from that of the parent imidazoline molecule tolazoline.

Pharmacologic and Molecular Discrimination of I2‐Imidazoline Receptor Subtypesa

“…3A, inset), has been shown to be an irreversible antagonist for α‐adrenoceptors and other unidentified receptors in rabbit iris muscles. 38 Because of the imidazoline structure of IBI and the high affinity of tolazoline for the two subtypes of I 2 ‐IR (K i = 130 nM for I 2A 39 and K i = 16 nM for I 2B 5 ), we reasoned that this compound could also alkylate these receptors. Preincubation experiments were performed to determine if IBI could irreversibly bind in vitro to I 2 ‐IR.…”

Comparison of post-junctional ?-adrenoceptors in iris dilator muscle of humans, and albino and pigmented rabbits

“…Under similar conditions, two concentration-response curves to PE at 1 h interval were reproducible (Ishikawa et al 1996).…”

Depolarization of membrane potential and the smooth muscle contraction by isothiocyanatobenzyl imidazoline in guinea-pig stomach circular muscle

“…Classical antagonists such as phentolamine (at 100 nmol/l), phenoxybenzamine, atropine and d-tubocurarine failed to block the contractile response to IBI (Venkataraman et al 1989;Lei et al 1995;Ishikawa et al 1996). On the other hand, the calcium channel antagonists verapamil and nifedipine blocked the contractile vascular response of the agonist.…”

“…On the isolated rabbit iris sphincter, IBI acts as an "irreversible agonist", i.e., the agonist produces contraction during the initial exposure, but the pre-exposed tissue fails to respond to the same agonist after washing. Nifedipine and phentolamine provide small protection against the blockade of phenylephrine in the iris dilator while IBI acts mainly as a non-specific irreversible blocker (Ishikawa et al 1996).…”