Shortening velocity is different in longitudinal and circular muscle layers of the rabbit urethra

The aim of the study was to investigate whether the functional difference b etween circular and longitudinal muscles in the female rabbit urethra is re flected in their shortening properties and lactate dehydrogenase (LDH) acti vity. For mechanical experiments the preparations were chemically skinned t o avoid influence of membrane-related mechanisms and to enable maximal acti vation. Force-velocity relations and the maximal shortening velocity (v(max )) were determined using the isotonic quick-release method. The v(max) was three times higher in longitudinal muscle. LDH isoform pattern was determin ed on agarose gels. The M-subunit, favourable for lactate formation, consti tuted 70% of the total in both types of muscle. There was no difference in the LDH isoform pattern despite the marked difference in v(max). We conclud e that the difference in v(max) reflects differences in the contractile mac hinery itself. These mechanical characteristics are advantageous for the ro le of the circular as a tonic muscle contracting during bladder filling, an d the longitudinal as a phasic muscle active in opening up the urethra duri ng micturition.