Porcine Stomach Smooth Muscle Force Depends on History-Effects

Figure 1. Representative picture of porcine stomach. Ventral view of external anatomy of a porcine stomach. (1) cardia, (2) fundus, (3) corpus, (4) antrum, (5) pylorus, (6) greater curvature, (7) smaller curvature. Black rectangle indicates longitudinal muscle strip dissected from the fundus. Longitudinal direction is marked by the black arrow.

Figure 2. Experimental protocol. Examination of varying ramp parameters followed a pseudorandomised block design (upper trace). The applied ramp-velocity and ramp-length are normalized to the maximum shortening velocity (νmax) and optimum muscle length (L0), respectively. The lower trace illustrates the fixed protocol of muscle contractions consisting of isokinetic ramp experiments (FE: force enhancement, FD: force depression), and isometric reference contractions (Isometric_Ref).

Figure 3. Section of a characteristic smooth tissue sample from porcine stomach. Representative picture from histological staining of samples from the proximal stomach (fundus) at 200% slack length (LS) in longitudinal direction. Four distinct layers can be identified: (1) tunica serosa, (2) longitudinal muscle layer (tunica muscularis), (3) transversal (circumferential) muscle layer (tunica muscularis), and (4) tela submucosa.

Figure 4. Force-length relationship. The length is normalized to optimum muscle length (L0, upper abscissa) and slack length (LS, lower abscissa), respectively. The force is normalized to maximum isometric force (Fim). Filled triangles and open circles indicate mean values of active and passive isometric smooth muscle forces, respectively. Bars indicate corresponding standard deviations. The ascending limb (f1(x); indicated by the solid black line) and descending limb (f2(x); indicated by the solid gray line) of the force-length relation (FLR) were fitted by linear regression models following the equations:f1(x) = 0.74x−0.47 and f2(x) = −0.51x+2.2, withx = L/LS. Isometric force-length measurements comprise n = 20 tissue samples up to lengths of 2.5 LS (corresponding to around 50% Fim). For determining the descending limb of the FLR for lengths longer than 2.5 LS, varying sample sizes were investigated (labeled with gray numbers below standard deviation- (s.d.) bars on the descending limb of the FLR).

Figure 5. Force-velocity relationship. Based on a series of isotonic contractions, the normalized force (means ± standard deviations are indicated by black bars) was determined as function of the velocity. The gray curve shows the typical hyperbolic shape observed by Hill (1938), with aFim=curv=0.36 (a describes the force asymptote) and νmax = 1.08 mm/s (the intersection of the fitted hyperbolic curve with the velocity-axis). Isotonic force-velocity measurements comprise n = 25 tissue samples. The force is normalized to maximum isometric force (Fim) and the velocity is expressed in absolute values [mm/s].

Figure 6. History-effects with varying ramp amplitudes. Representative force-time (upper graph) and length-time traces (lower graph) with isokinetic length changes comprising three different ramp amplitudes (5, 7, 10% L0) at constant velocity of 40% νmax (strip number S40). The force is normalized to maximum isometric force (Fim) and length to optimum muscle length (L0). Force enhancement (FE, difference between black arrows) and force depression (FD, difference between white arrows) are the force difference between ramp experiments (solid lines) and isometric reference contraction (dotted line) determined 10 s after the end of the ramp, shown exemplarily for the longest (10% L0) ramp.

Figure 7. History-effects with varying ramp velocities. Representative force-time- (upper graph) and length-time traces (lower graph) with isokinetic length changes comprising three different ramp velocities (20, 40, 70% νmax) at constant ramp amplitude of 7% L0 (strip number S38); numbers without units indicate velocity in percent of maximum shortening velocity [% νmax]. The force is normalized to maximum isometric force (Fim) and length to optimum muscle length (L0). Force enhancement (FE, difference between black arrows) and force depression (FD, difference between white arrows) are the force difference between ramp experiments (solid lines) and isometric reference contraction (dotted line) determined 10 s after the end of the ramp, shown exemplarily for the fastest (70% νmax) ramp.

Figure 8. Dependency of history-effects (FE and FD) on ramp length (A) and on ramp velocity (B). Mean forces ± standard deviations are given. Forces are normalized to the forces obtained during purely isometric reference contraction (in percent of maximum isometric force [% F/Fim]) at optimum muscle length [L0] (indicated by horizontal black solid lines). (A) Ramp length was varied (black: 5% L0, dark gray: 7% L0, light gray: 10% L0) at constant ramp velocity (40% νmax). (B) Ramp velocity was varied (black: 20% νmax, dark gray: 40% νmax, light gray: 70% νmax) at constant ramp length (7% L0). Brackets and asterisks (*) mark differences in forces after stretch (eccentric) and shortening (concentric) in the intergroup comparison. Significance levels are marked as follows:*P < 0.05, **P < 0.01, and ***P < 0.001.