Abstract The ability of a series of novel imidazoline (IMID) compounds (fluoryl-, methoxy- and methyl-phenyl derivatives of clonidine) to inhibit the vasorelaxation and hyperpolarisation response to exogenous K + (1–10 mM) was assessed in the rat middle cerebral artery (MCA) us- ing the small vessel myograph. In this preparation, K + -in- duced relaxation was inhibited by low concentrations of Ba 2+ (30 μM) but not affected by the Na + /K + ATPase in- hibitor ouabain (10 μM), or a combination of tetraethyl- ammonium (TEA; 1 mM), 4-aminopyridine (1 mM) and glibenclamide (10 μM). These results are consistent with K + eliciting a vasorelaxation response through the activa- tion of inwardly rectifying K + channels (Kir channels) in this tissue. K + -mediated vasorelaxation was assessed in the absence and in the presence of two concentrations of the IMID compounds (1 and 10 μM). The majority of the compounds investigated caused marked inhibition of K + -mediated relaxation at these concentrations. In electro- physiological studies the fluoryl-derivative (IMID-4F; 10 μM) potently inhibited the hyperpolarisation response that accompanies the relaxation response to exogenous K + . In conclusion, we have identified a number of IMID compounds that inhibit relaxation and hyperpolarisation responses mediated via Kir channels in the rat MCA. Many of these compounds have a greater potency as in- hibitors of Kir channels than Ba 2+ , and may be a useful tool in studying Kir channel function. Keywords Imidazoline · K + channels · Kir channel · Vasorelaxation · Cerebral artery Abbreviations 5-HT 5-hydroxytryptamine · IMID imidazoline · MCA middle cerebral artery · OB ouabain · SUR sulphonylurea receptor · pK B –log([antagonist concentration {M}]/[Concentration ratio 1]) · TEA tetraethylammonium · TPP tetraphenylphosphonium Introduction The inwardly rectifying K + channel (Kir) is found in a va- riety of tissues (Doupnik et al. 1995) and has numerous subtypes denoted Kir1–7. These channels can exist as channels in their own right, and also as part of channel-re- ceptor complexes (Alexander and Peters 1999). For ex- ample, the K ATP channel is known to be a complex of a sulphonylurea receptor (SUR) and an inward rectifier K + channel (Kir 6.1 or Kir 6.2) (Aguilar-Bryan et al. 1995). Our laboratory has been developing novel compounds that modulate K ATP channel activity. Commonly used se- lective inhibitors of the K ATP channel, such as gliben- clamide, act at sites on the SUR (Ashcroft and Gribble 1998). We have previously described two classes of com- pounds that also antagonise K ATP channels. These are qua- ternary ions such as tetraphenylphosphonium (Piekarska and McPherson 1997; Zhang et al. 1998) and also novel imidazoline (IMID) compounds such as IMID-4F (McPher- son et al. 1999; Bell et al. 2000). We have previously sug- gested (Zhang et al. 1998; Bell et al. 2000) that these com- pounds inhibit the K ATP channel via a site different to that of glibenclamide; possibly on the Kir subunit. Another study has shown that the imidazoline compound phentol- amine blocks the K ATP channel by interacting with the pore of the Kir channel component of the K ATP channel (Proks and Ashcroft 1997). If imidazoline and quaternary- based compounds do act at the Kir component of the K ATP channel this raises the possibility that they might also in- hibit Kir responses mediated through other subtypes of Kir. It has previously been shown that Kir channels are in- volved in the regulation of vascular smooth muscle mem- brane potential and tone, particularly in small, resistance- like vessels (Edwards et al. 1988; Knot et al. 1996; John- son et al. 1998). Rat isolated cerebral vessels contain Kir channels, reported to be of the sub-type Kir2.1 (Bradley et Joanne L. Favaloro · Karen L. Andrews · Grant A. McPherson Novel imidazoline compounds that inhibit Kir-mediated vasorelaxation in rat middle cerebral artery Naunyn-Schmiedeberg’s Arch Pharmacol (2003) 367 : 397–405 DOI 10.1007/s00210-003-0693-8 Received: 2 October 2002 / Accepted: 8 January 2003 / Published online: 27 February 2003 ORIGINAL ARTICLE J. L. Favaloro (✉) · K. L. Andrews · G. A. McPherson Department of Pharmacology, Monash University, 3800 Clayton, Victoria, Australia Tel.: +61-3-99054856, Fax: +61-3-99055851, e-mail: Joanne.Favaloro@med.monash.edu.au © Springer-Verlag 2003