Adaptation of the length-active tension relationship in rabbit detrusor

Abstract

Studies have shown that the length-tension (L-T) relationships in airway and vascular smooth muscles are dynamic and can adapt to length changes over a period of time. Our prior studies have shown that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that can shift along the length axis as a function of strain history and activation history. The present study demonstrates that the active L-T curve for DSM is also dynamic and that the peak active tension produced at a particular muscle length is a function of both strain and activation history. More specifically, this study reveals that the active L-T relationship, or curve, does not have a unique peak tension value with a single ascending and descending limb, but instead reveals that multiple ascending and descending limbs can be exhibited in the same DSM strip. This study also demonstrates that for DSM strips not stretched far enough to reveal a descending limb, the peak active tension produced by a maximal KCl-induced contraction at a short, passively slack muscle length of 3 mm was reduced by 58.6 +/- 4.1% (n = 15) following stretches to and contractions at threefold the original muscle length, 9 mm. Moreover, five subsequent contractions at the short muscle length displayed increasingly greater tension; active tension produced by the sixth contraction was 91.5 +/- 9.1% of that produced by the prestretch contraction at that length. Together, these findings indicate for the first time that DSM exhibits length adaptation, similar to vascular and airway smooth muscles. In addition, our findings demonstrate that preconditioning, APS and adaptation of the active L-T curve can each impact the maximum total tension observed at a particular DSM length.

Fig. 1.

A: tension measurement protocol diagram. Detrusor smooth muscle (DSM) strips were incubated for 2 min in 0Ca, stretched or released to the muscle length of interest, if necessary; incubated for an additional 2 min in 0Ca; incubated in normal physiological salt solution (NPSS) for 3 min, and then incubated in NPSS modified to include 110 mM KCl, substituted isosmotically for NaCl (KPSS) for 1 min to induce a contraction. “Passive” tension (Tp) was taken to be the lower value of either the final tension in NPSS (Tp1, path 1, solid line) or, if active tension developed during the 3-min period in NPSS, the final tension in 0Ca (Tp2, path 2, dashed line). Active tension (Ta1 or Ta2) was calculated by subtracting the passive tension (either Tp1 or Tp2) from the peak total KPSS-induced tension (T_peak). B: tension data from 2 DSM strips illustrating the tension measurement protocol. Strip 1 (black line) exhibited less tension in NPSS (F) than in 0Ca (C), while strip 2 (gray line) exhibited more tension in NPSS (G) than in 0Ca (D) because it developed spontaneous contractile rhythm in NPSS (E).

Fig. 2.

Rationale for 3-min NPSS treatment following incubation of tissues in 0Ca and preceding stimulation in KPSS. A and B: Tp (A) and KCl-induced Ta (B) values for 3 isometric KCl-induced contractions performed at 9 mm using the timing from Fig 1, except that tissues were not incubated in NPSS before stimulation in KPSS for contractions 1 and 3. Therefore, the DSM strips were incubated in 0Ca for 7 min total before the and contractions 1 and 3 (white bars) and in 0Ca for 4 min and then NPSS for 3 min before the contraction 2 (gray bars). Data are normalized to the maximum (optimal) active tension (T_o) for that tissue (means ± SE; n = 3). *P < 0.05 compared with 0Ca.

Fig. 3.

A: length-tension (L-T) curve protocol for tissues that were not preconditioned. Protocol consisted of 3 series (S1–S3) of 5 measurements at increasing lengths from 3 to 15 mm. B–F: L-Ta curves (means ± SE) for S1 (white symbols), S2 (black symbols), and S3 (gray symbols in C only) and corresponding L-Tp curves (S1, dotted line; S2, dashed-dotted line; S3, dashed line in C only). Data from 12 experiments were grouped according to L_o1 and normalized to T_o1 (B–D) or Ta_max (E). For S1–S3, peak (optimal) Ta values are labeled T_o1–T_o3 and correspond to optimal lengths L_o1 (6 mm, B; 9 mm, C; 12 mm, D; and ≥15 mm, E); L_o2 (12 mm, B and C, ≥15 mm, D and E); and L_o3 (12 mm, C). B: L-Ta and L-Tp curves for S1 and S2 for tissues with L_o1 at 6 mm (n = 3). C: L-Ta and L-Tp curves for S1–S3 for tissues with L_o1 at 9 mm (n = 3). D: L-Ta and L-Tp curves for S1 and S2 for tissues with L_o1 at 12 mm (n = 4). E: L-Ta and L-Tp curves for S1 and S2 for tissues with L_o1 ≥15 mm (n = 2). F: L-Ta and L-Tp curves for S1 and S2 for all 12 tissues (n = 12). B–F: *P < 0.05, Ta for S2 was significantly less than Ta for S1 at that particular length (paired t-test). C: ΨP < 0.05, Ta for S3 was significantly less than Ta for S2 at that particular length (paired t-test).

Fig. 4.

A: L-T curve protocol consisting of 2 series measurements (S1 and S2) to determine if the L-Tt curve shifts due to preconditioning. B: L-Tt curves for S1 (solid line, open symbols) and S2 (dashed line, solid symbols). *P < 0.05 Tt for S2 was significantly less than Tt for S1 at that particular length (paired t-test, n = 4). C and D: full (C) and zoomed (D, box from C) L-Ta curves. Localized peak Ta values for S1 and S2 are labeled T_o1a (C), T_o1b (D), and T_o1c (D) and T_o2a (C and D), T_o2b (D), and T_o2c (D). C: L-Tp curves for S1 (solid line) and S2 (dotted line). *P < 0.05 Ta for S2 was significantly less than Ta for S2 at a particular length <9 mm (paired t-test, n = 4). D: at 10.75 and 11 mm, Ta values for S1 were not different from the corresponding values for S2 (paired t-test, P > 0.05, n = 4). The second Ta values at 10.5 mm were significantly greater than the first for both S1 and S2 (ΨP < 0.05, paired t-test, n = 4), forming counterclockwise loops (arrow) suggesting adaptation to that length region. E: changes in Tt (solid line), Ta (dashed line), and Tp (dashed-dotted line) between S1 and S2 for lengths between 3 and 10.5 mm. The change in Tt between S1 and S2 was due to the change in Ta at shorter lengths and due to the change in Tp at longer lengths.

Fig. 5.

A: protocol to identify adaptation of the L-Ta curve at 3 mm consisting of 2 series of stretches and tension measurements between 3 and 9 mm (S1 and S2) followed by a series of 6 tension measurements at 3 mm and for some tissues measurements at 6 and 9 mm (S3). B–D: L-Ta (solid lines) and L-Tp (dotted lines) curves for S1 (diamonds), S2 (triangles), and S3 (squares) (±SE at 6 and 9 mm). Data were categorized according to the shape of the L-Ta curve produced by S1 and normalized to Ta_max. B: data for tissues producing at least 10% greater Ta at 9 than at 6 mm during S1 were categorized as tissues with a “more steep ascent 6–9 mm” (n = 9 for data points 1–13 and n = 8 for data points 14 and 15). C: data for tissues producing <10% greater Ta at 9 than at 6 mm during S1 were categorized as tissues with a “less steep ascent 6–9 mm” (n = 6, data points 1–13 only). D: data for tissues producing greater Ta at 6 than at 9 mm during S1 were categorized as tissues with a “descent 6–9 mm” (n = 5 for data points 1–13 and n = 3 for data points 14 and 15). D: data for tissues incubated in NPSS for 7 min during each tension measurement cycle, instead of 4 min in 0Ca and 3 min in NPSS (see Figs 1–2) were categorized as tissues with an “ascent 6–9 mm, no 0Ca” (n = 4). F: Ta values (±SE) for contractions at 3 mm, corresponding to the data in B–E, for S1 (dark gray), S2 (white), and S3 (light gray), along with the increase in Ta from contractions 8 to 13 (T_adapt, “13–8”, calculated as Ta for 13 minus Ta for 8). F: * NS indicates whether Ta for contraction 8 or 13 at 3 mm was (*) or was not (NS) significantly different from the Ta value for contraction 2 at 3 mm for that category (ANOVA, Newman-Keuls multiple comparison test), and Ψ indicates T_adapt (“13–8”) was not different from T_adapt for the other categories (ANOVA, Newman-Keuls multiple comparison test).

Fig. 6.

A: protocol to determine if Ta increased as much following incubation in NPSS as following multiple KCl-induced contractions. Control tissues were subjected to 3 sets of 3 8-min tension measurement cycles at L_ref alternated with 3 sets of 4 cycles at 0.8L_ref. For “test” tissues, cycles 12–14 and 20–21 at 0.8L_ref were replaced with 24- and 16-min incubations in NPSS, respectively. B: Ta values for the first and fourth cycles of each series of measurements at 0.8L_o were normalized to Ta for the first contraction at 0.8L_o, Ta_5 (data point 5, ±SE). Ta increased during initial control contractions in both groups (8 vs. 5, *P < 0.05 compared with 1). Ta did not increase following incubation in NPSS for 24 min (15 vs. 5, test, n = 5), but did increase following 3 isometric contractions (15 vs. 12 and 5, control, n = 4). C: Ta adapted (increased) following initial control contractions in both groups (8 minus 5, ΩP < 0.05 compared with 0). However, Ta did not significantly adapt following 16 min in NPSS but did adapt following KCl-induced contractions (22 minus 19, test vs. control, ΩP < 0.05 compared with 0).

Fig. 7.

Normalized L-Ta and L-Tp curves. A: data from S1 in Fig 3C normalized to T_o1 and L_o1 (Ta/T_o1, ♦), data approximated from Ref. normalized to T_o and L_o (□), and data approximated from Ref. normalized to T_o and L_o (○). B: data from S1 in Fig 3C normalized to T_o1 and L_o1 (Ta/T_o1, ♦) and data from S2 in Fig 3C normalized to T_o1 and L_o1 (Ta/T_o1, ●, dashed line) and normalized to T_o2 and L_o2 (Ta/T_o2, ▲).