Contractile and endurance properties of feline triangularis sterni muscle

Variations in respiratory muscle activity during echolocation when stationary in three species of bat (Microchiroptera: Vespertilionidae)

Echolocating bats use respiratory muscles to power the production of biosonar vocalisations. The physical characteristics of these calls vary among species of bat, and variations also exist in the timing and patterns of respiratory muscle recruitment during echolocation. We recorded electromyograms from the respiratory muscles of three species of bat (Family Vespertilionidae) while the animals vocalised from stationary positions. Activity was recorded consistently from the lateral abdominal muscles (internal abdominal oblique and transversus abdominis) from all calling bats, but we found much variation within and among species. Bats in the family Vespertilionidae devoted longer periods of expiratory muscle activity to each call than did the mormoopid bat Pteronotus parnellii. These differences correlate negatively with the duration of calls. We suggest that morphological adaptations in some bats may facilitate the economic production of echolocation calls at rest.

Peptide toxin blockers of voltage-sensitive K+ channels: inotropic effects on diaphragm

Agents that block many types of K+ channels (e.g., the aminopyridines) have substantial inotropic effects in skeletal muscle. Specific blockers of ATP-sensitive and Ca2+-activated K+ channels, on the other hand, do not, or minimally, alter the force of nonfatigued muscle, consistent with a predominant role for voltage-gated K+ channels in regulating muscle force. To test this more directly, we examined the effects of peptide toxins, which in other tissues specifically block voltage-gated K+ channels, on rat diaphragm in vitro. Twitch force was increased in response to alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha (17 +/- 6, 22 +/- 5, 42 +/- 14, and 13 +/- 5%; P < 0.05, < 0.01, < 0.05, < 0.05, respectively) but not in response to delta-dendrotoxin or BSA (in which toxins were dissolved). Force during 20-Hz stimulation was also increased significantly by alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha. Among agents, increases in twitch force correlated with the degree to which contraction time was prolonged (r = 0.88, P < 0.02). To determine whether inotropic effects could be maintained during repeated contractions, muscle strips underwent intermittent 20-Hz train stimulation for a duration of 2 min in presence or absence of gamma-dendrotoxin. Force was significantly greater with than without gamma-dendrotoxin during repetitive stimulation for the first 60 s of repetitive contractions. Despite the approximately 55% higher value for initial force in the presence vs. absence of gamma-dendrotoxin, the rate at which fatigue occurred was not accelerated by the toxin, as assessed by the amount of time over which force declined by 25 and 50%. These data suggest that blocking voltage-activated K+ channels may be a useful therapeutic strategy for augmenting diaphragm force, provided less toxic blockers of these channels can be found.

Pharyngeal dilator muscle contractile and endurance properties in neonatal piglets

Pharyngeal dilator muscles are critical for maintaining upper airway patency in the neonatal period. The present study examined in vitro the contractile properties of a pharyngeal dilator muscle, the sternohyoid, in 1-7-day-old piglets (n = 24). Isometric contraction and half-relaxation times were 36.7 +/- 1.1 and 30.9 +/- 1.2 msec, respectively. Twitch potentiation ('staircase phenomenon') and post-tetanic potentiation were noted following repetitive stimulation. During prolonged repetitive stimulation with a standard (40 Hz) fatigue test, muscle force declined gradually over time, with loss of half of the initial force occurring over 138 +/- 11 sec, and a 2-min fatigue index (ratio of force at 2 min to initial force) of 0.52 +/- 0.03. An additional 10 piglets were studied at ages of 14-20 days. Muscle from older piglets had comparable isometric twitch kinetics as that of younger animals. However, sternohyoid muscle from the older piglets had worse endurance than muscle from the younger animals, as indicated by a shorter time required for force to decrease by half (86 +/- 10 sec, P < 0.01) and a lower 2-min fatigue index (0.36 +/- 0.03, P < 0.01). These data indicate that for the sternohyoid muscle of the newborn piglet (a) physiological properties are consistent with moderate to fast contraction with good endurance, (b) force potentiates during repetitive twitch stimulation and following a brief period of tetanic stimulation, and (c) there is worsening of endurance but no change in isometric twitch kinetics with increasing age during the first weeks of life.