The proposed research project is designed to further study changes seen in adrenergic sensitivity (i.e. increased alpha response at low temperature and increased beta response at high temperatures) of rabbit iris muscle in vitro previously demonstrated with changes in temperature. Muscle tension will be monitored and dose response relationships for adrenergic agonists (norepinephrine and isoproterenol) before and after adrenergic antagonists (propranolol, sotalol, phentolamine and phenoxybenzamine) will be compared. Specifically, the responses of the iris constrictor muscle to adrenergic agents will be studied using iris dilator and constrictor muscle; also, tissues from thyroidectomized animals and thyroidectomized animals treated with thyroxin or triiodothyronin will be tested to determine whether receptor interconversion occurs under these conditions. It is proposed that receptor interconversion or sensitivity change can explain changes in responsiveness of the iris muscle in certain disease states.