Although abnormal myocardial calcium homeostasis in the cardiomyopathic hamster (CMH) has been documented in the hypertrophic stage of the disease, the Ca2+ tolerance before the hypertrophic stage has not been investigated. We studied isovolumic contractile function in response to a variety of Ca2+ stresses including increases in perfusate [Ca2+] (Cao), the Ca2+ channel agonist Bay K 8644, and alpha- or beta-adrenergic agonists of isolated perfused hamster hearts from 24-45-day-old male CMH, BIO 14.6 strain, and age- and sex-matched F1B strain controls. The coronary flow at a constant perfusion pressure did not differ between two groups at baseline or after any Ca2+ stress. At a Cao of 1.0 mM, neither end-diastolic pressure (EDP) nor developed pressure (DP) nor half relaxation time (RT1/2) during stimulation at 1-3 Hz differed between the two groups; as Cao was increased up to 10 mM, CMH hearts showed a lower threshold for the occurrence of a Ca2+ overload profile: EDP and RT1/2 increased to a greater, and DP to a lesser, extent in CMH than in control hearts. To determine whether calcium influx via Ca2+ channels mediates the lower threshold for Ca2+ overload in CMH hearts, we measured resting pressure and scattered laser light intensity fluctuation (SLIF) in unstimulated hearts. Prior studies have shown that SLIF is generated by microscopic tissue motion caused by diastolic spontaneous sarcoplasmic reticulum Ca2+ release and that SLIF amplitude reflects the extent of cell and sarcoplasmic reticulum Ca2+ loading. The Ca(2+)-dependent increase in resting pressure in unstimulated hearts was highly correlated with an increase in SLIF, and this relation was steeper in CMH than in control hearts. CMH hearts also showed a reduced threshold for the occurrence of a Ca2+ overload profile in response to the adrenergic receptor agonists and the Ca2+ channel agonist during electrical stimulation in a Cao of 2.0 mM: maximum DP achieved with each agonist was significantly less and the dose-response curves to each agonist were shifted leftward in CMH versus control hearts. In CMH hearts EDP began to increase at a significantly lower concentration of each agonist, and the maximum extent of increase in EDP in response to all agonists was significantly enhanced compared with control hearts. In response to beta-adrenergic or Ca2+ channel agonists, neither resting pressure nor SLIF in unstimulated hearts increased in control or in CMH hearts. In contrast, in response to alpha-adrenergic stimulation, both SLIF and resting pressure increased to a greater extent in CMH than in control hearts.(ABSTRACT TRUNCATED AT 400 WORDS)