The whistle and the rattle: The design of sound producing muscles (rattlesnakeytoadfishycalcium transientsymuscle mechanicsyswimbladder)

TLDR: It is found that to generate the "boatwhistle" mating call, the swimbladder muscle fibers of toadfish have evolved a large and very fast calcium transient, a fast crossbridge detachment rate, and probably a fast kinetic off-rate of Ca2+ from troponin.

Abstract: Vertebrate sound producing muscles often operate at frequencies exceeding 100 Hz, making them the fastest vertebrate muscles. Like other vertebrate muscle, these sonic muscles are ''synchronous,'' necessitating that calcium be released and resequestered by the sarcoplasmic reticulum during each contraction cycle. Thus to operate at such high frequencies, vertebrate sonic muscles require extreme adap- tations. We have found that to generate the ''boatwhistle'' mating call ('200 Hz), the swimbladder muscle fibers of toadfish have evolved (i) a large and very fast calcium transient, (ii) a fast crossbridge detachment rate, and (iii) probably a fast kinetic off-rate of Ca 21 from troponin. The fibers of the shaker muscle of rattlesnakes have independently evolved similar traits, permitting tail rattling at '90 Hz. Skeletal muscle fibers perform a wide range of activities, and different fiber types are accordingly designed to operate at different speeds and frequencies (1). A number of modifica- tions appear to underlie this diversity. For example, in loco- motory muscle, compared with slow twitch fibers, fast twitch fibers have a faster myosin with a higher maximum velocity of shortening (Vmax) (2, 3), a greater content of sarcoplasmic reticulum (SR), and its associated Ca 21 pumps (4, 5), a different isoform of the SR Ca 21 pump (SERCA1 in fast versus SERCA2 in slow) (6, 7) and a greater concentration of parvalbumin (a soluble protein that binds both calcium and magnesium) (5, 8). There is also evidence that fast fibers have a briefer myoplasmic free Ca 21 concentration ((Ca 21 )) tran- sient (9, 10) and less sensitive force-pCa relationship (11, 12). To understand the physiological modifications that underlie very rapid contractions, we have studied two of the fastest vertebrate muscles known. Both of these ''sonic'' muscles are used to produce sounds at the frequency at which the muscle contracts. The ''boatwhistle'' mating call of the male toadfish (Opsanus tau) is generated by '200 Hz contractions (25°C) of the muscles encircling the fish's gas-filled swimbladder (13- 15). The familiar ''rattle'' of the venomous western diamond- back rattlesnake (Crotalus atrox) is generated by '90 Hz contractions (35°C) of the shaker muscles at the base of the tail (16-19). The operational frequencies of these sonic muscles are 1-2 orders of magnitude higher than those of the loco- motory muscles in the same animals (0.5-5 Hz) (20). For fibers to operate at such high frequencies, they must activate and relax rapidly. Based on morphological and bio- chemical evidence from swimbladder of a very large SR Ca 21 pump density, a high density of SR Ca 21 release sites, and a large parvalbumin concentration, it has been proposed that an important modification of these sonic fibers is unusually rapid Ca 21 cycling (5, 21). By measuring myoplasmic free (Ca 21 ), we found that these fibers do indeed have unusual Ca 21 tran- sients—in fact the largest and fastest ever recorded. How- ever, our results showed that a fast Ca 21 transient alone is not sufficient for high frequency operation. By measuring Vmax, an index of crossbridge detachment rate, and the force-pCa relationship in skinned fibers, a possible index of troponin kinetics, we found that rapid activation and relax- ation likely also require a modification of the crossbridge kinetic rate, and probably a modification of the kinetics of Ca 21 -troponin binding. In reaching these conclusions, we first compared the above measurements in three fiber types from toadfish, ranging from slow twitch swimming fibers to the superfast twitch swimbladder fibers. We then compared the properties of rattlesnake shaker fibers with those of swimbladder.