Force-frequency (FFR) and relaxation-frequency relationships (RFR) and post-extrasystolic potentiation (PESP) act as physiologic mirrors reflecting changes in Ca2+ release and uptake by sarooplasmic reticulum (SR). It has been proposed that SR is relatively immature in neonatal animal and FFR and RFR may be impaired in them. We studed certain new aspects of FFR and RFR in 4 neonatal goats (Age: 4 weeks) and 3 adult goats. Left ventricular (LV) dP/dtmax and time constant of relaxation (τ) were calculated from micromanometric LV pressure tracings at increasing heart rates (HR) generated by right ventricular pacing. The phenomenon of PESP was studied by pacing at a constant cycle length(400ms) and by introducing an extrasystole and post-extrasystole at cycle lengths of 250ms and 600 ms, respectively. Both FFR(dP/dtmax vs HR) and RFR (τ vs HR) have biphasic relationships separated by a critical HR at which this transition occurs. Critical HR was nearly identical for both FFR and RFR. Mean critical HR was higher in neonatal vs adult goats (250± 35 vs 180±22 beats/min, p, 0.05). Following ryanodine (a SR Ca2+-release channel blocker), critical HR was shifted to the left in both neonatal and adult goats (170±34 vs 140±28 beats/min). Fig. 1

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Force-frequency (A)and relaxation-frequency (B) relationships in neonatal goats

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In neonatal goats there was no PESP. However, post-extrasystolic beats produced prolongation of τ (impaired relaxation.) suggesting impaired reuptake of Ca2+. After ryanodine, a post-extrasystolic beat either produced shortening of τ or had no effect, suggesting low to normal cytosolic Ca2+- levels Conclusions: FFR and RFR remain intact in neonatal goats. The unique physiologic relationships studied here act as mirrors and reflect changes in cytosolic Ca2+ and functions of Ca2+-cycling proteins of the SR (ryanodine, Ca2+-pump, phospholamban) and sarcolemma (Na-Ca2+-exchanger).