Norepinephrine Turnover in the Heart and Spleen of the Cardiomyopathic Syrian Hamster

Although a reduction in myocardial norepinephrine stores in cardiac hypertrophy and congestive failure is well documented, norepinephrine turnover has been inadequately studied in such hearts. We compared norepinephrine turnover in control and cardiomyopathic hamsters by following the decline in specific activity of myocardial norepinephrine after labeling with an intraperitoneal tracer dose of H-norepinephrine. Adult myocardial norepinephrine concentrations were not attained until 4 weeks of age in both strains. There was no difference in the rate constant (K) for myocardial norepinephrine turnover (0.107 ± 0.004 hours vs. 0.100 ± 0.005 hours) in the two strains of hamsters during the neonatal period. In young control hamsters, K fell to 0.064 ± 0.004 hours, but that for age-matched hamsters with mild cardiac hypertrophy was 0.102 ± 0.001 hours (P < 0.001). There was little change in K as control hamsters aged. With the development of more severe hypertrophy in cardiomyopathic hamsters, cardiac norepinephrine decreased and resting K rapidly increased to approach the value obtained when hamsters were subjected to immobilization stress (0.302 ± 0.013 hours). The maximum achievable K remained the same for both control and dystrophic hamsters even during terminal disease. Prolonged immobilization led to a reduction in cardiac norepinephrine in both strains. Ganglionic blockade of failing hamsters completely restored the levels of both cardiac norepinephrine and K to control values. Splenic noradrenergic nerves showed no change in K, norepinephrine content, or maximum K during cardiac decompensation. We conclude that, in the late stages of hamster cardiomyopathy, there is a progressive and possibly specific increase in cardiac sympathetic tone which leads to a concomitant decrease in cardiac norepinephrine. With the loss of sympathetic reserve, congestive failure supervenes.