>Hi, it's a good question and there are probably a few possible answers. Firstly, as has been shown in other tissues (i.e. brain, liver, spleen) the receptor number is significantly lower. If alpha1B-AR's are involved in the response to field stimulation then perhaps a reduced receptor number may be responsible for the difference. It has been suggested that alpha1B-AR's are confined to the synaptic region and so stimulation of extrajunctional (alpha1A?) receptors will require higher frequency stimulation. Alternatively, I think it is more likely to be the result of the alpha1A- and alpha1B-AR's having different Ca2+ coupling mechanisms. If the alpha1B- is more associated with release of intracellular Ca2+ then the absence of that component may explain why the response is slower (i.e. it is more dependent on Ca2+ influx). We have a lot of work still to do. I'd be happy to hear any explanation you may have. Thanks for you interest.