Abstract
The length-tension relationship and the effect of paired electrical stimulation were studied in right ventricular papillary muscles obtained from cats. The animals were divided into three age groups: neonatal kittens, less than 24 hr of age; infant kittens, 16--18 days of age; and adult cats. 2. The muscles were electrically driven to contract isometrically using field stimulation at 10--20% above the threshold valus. 3. At Lmax the neonatal preparations produced significantly less developed (active) tension than did the adult preparations. The resting tension at Lmax was significantly greater in the neonatal preparations than in the adult ones. The infant group occupied an intermediate position between the two other groups. 4. The response to paired electrical stimulation was less in the neonatal group than in either the infants or the adults. 5. It is concluded that post-natal developmental changes occur in the contractile properties of the myocardium of the cat.