It has been proposed that Ca2⁢-activated K⁢ (KCa) channels play an essential role in vascular tone. The alterations of the properties of coronary KCa channels have not been studied as a possible mechanism for impaired coronary reserve in cardiac hypertrophy. The present studies were carried out to determine the properties of coronary KCa channels in normal and hypertrophied hearts. These channels were measured from rabbit coronary smooth muscle cells using a patch clamp technique. The main findings of the present study are as follows: (1) the unitary current amplitudes and the slope conductance of coronary KCa channels were decreased without changes of the channel kinetics in isoproterenol-induced cardiac hypertrophy; (2) the sensitivity of coronary KCa channels to the changes of intracellular concentration of Ca2⁢ was reduced in isoproterenol-induced cardiac hypertrophy. From above results, we suggest for the first time that the alteration of KCa channels are involved in impaired coronary reserve in isoproterenol-induced cardiac hypertrophy.