This dissertation focuses on the signal transduction mechanism of {dollar}\beta\sb1{dollar}- and {dollar}\beta\sb2{dollar}-adrenoceptor ({dollar}\beta\sb1{dollar}AR and {dollar}\beta\sb2{dollar}AR) subtypes and their modulatory effects on cardiac excitation-contraction coupling. We demonstrated that both {dollar}\beta\sb1{dollar}AR and {dollar}\beta\sb2{dollar}AR functionally coexist in rat ventricular myocytes but that stimulation of these receptor subtypes elicits qualitatively different cellular responses. {dollar}\beta\sb2{dollar}AR stimulation induced by zinterol (ZINT) or by isoproterenol in the presence of the selective {dollar}\beta\sb1{dollar}AR antagonist, CGP 20712A, increased contraction amplitude to about the same extent as {dollar}\beta\sb1{dollar}AR stimulation induced by norepinephrine (NE). {dollar}\beta\sb1{dollar}AR stimulation has a potent effect to abbreviate the durations of the contraction and Ca{dollar}\sb{lcub}\rm i{rcub}{dollar} transient; in contrast, {dollar}\beta\sb2{dollar}AR stimulation has only a minor relaxation effect. In addition, {dollar}\beta\sb1{dollar}AR, but not {dollar}\beta\sb2{dollar}AR stimulation, increases the diastolic Ca{dollar}\sp{lcub}2+{rcub}{dollar} level and evokes spontaneous Ca{dollar}\sb{lcub}\rm i{rcub}{dollar} oscillations. {dollar}\beta\sb1{dollar}AR and {dollar}\beta\sb2{dollar}AR also differ in their effects on I{dollar}\sb{lcub}\rm Ca{rcub}{dollar}: whereas both {dollar}\beta\sb1{dollar}AR and {dollar}\beta\sb2{dollar}AR stimulation increase the peak I{dollar}\sb{lcub}\rm Ca{rcub}{dollar} amplitude to a similar extent, only {dollar}\beta\sb2{dollar} markedly prolongs the I{dollar}\sb{lcub}\rm Ca{rcub}{dollar} inactivation time; accordingly, {dollar}\beta\sb2{dollar}AR stimulation prolongs the action potential duration to a greater extent than {dollar}\beta\sb1{dollar}AR stimulation. Subsequent studies in rat ventricular myocytes have demonstrated that while both {dollar}\beta\sb1{dollar}AR and {dollar}\beta\sb2{dollar}AR stimulation increase total cellular cAMP to a similar extent, the effects of {dollar}\beta\sb2{dollar}AR stimulation on Ca{dollar}\sb{lcub}\rm i{rcub}{dollar} transient and contraction are largely dissociated from the cAMP increase. In addition, {dollar}\beta\sb2{dollar}AR stimulation does not result in phosphorylated phospholamban to the same extent as does {dollar}\beta\sb1{dollar}AR stimulation. These results suggest that {dollar}\beta\sb2{dollar}AR might be coupled to signaling pathway(s) other than the G{dollar}\sb{lcub}\rm s{rcub}{dollar}-mediated activation of adenylyl cyclase. This is further supported by the observation that pertussis toxin (PTX) pretreatment specifically potentiates the {dollar}\beta\sb2{dollar}AR evoked increases in I{dollar}\sb{lcub}\rm Ca{rcub}{dollar}, Ca{dollar}\sp{lcub}2+{rcub}{dollar} transient and contraction but not that of {dollar}\beta\sb1{dollar}AR stimulation. Thus, in the absence of PTX, substantial coupling occurs between {dollar}\beta\sb2{dollar}AR and a PTX-sensitive G-protein, exerting a negative feedback on the cardiac responses to {dollar}\beta\sb2{dollar}AR stimulation. Apparently, the distinct {dollar}\beta{dollar}AR subtype actions reside at least in part, in the different receptor-G protein interactions. The activation of more than a single G protein during {dollar}\beta\sb2{dollar}AR stimulation, leading to functionally opposite effects, may provide a mechanism to protect the heart from Ca{dollar}\sp{lcub}2+{rcub}{dollar} overload and arrhythmias during the response to stress.