Small Ca 2+ releases enable hour-long high-frequency contractions in midshipman swimbladder muscle.

Type I males of the Pacific midshipman fish ( Porichthys notatus ) vibrate their swimbladder to generate mating calls, or "hums," that attract females to their nests. In contrast to the intermittent calls produced by male Atlantic toadfish ( Opsanus tau ), which occur with a duty cycle (calling time divided by total time) of only 3-8%, midshipman can call continuously for up to an hour. With 100% duty cycles and frequencies of 50-100 Hz (15°C), the superfast muscle fibers that surround the midshipman swimbladder may contract and relax as many as 360,000 times in 1 h. The energy for this activity is supported by a large volume of densely packed mitochondria that are found in the peripheral and central regions of the fiber. The remaining fiber cross section contains contractile filaments and a well-developed network of sarcoplasmic reticulum (SR) and triadic junctions. Here, to understand quantitatively how Ca2+ is managed by midshipman fibers during calling, we measure (a) the Ca2+ pumping-versus-pCa and force-versus-pCa relations in skinned fiber bundles and (b) changes in myoplasmic free [Ca2+ ] (Δ[Ca2+ ]) during stimulated activity of individual fibers microinjected with the Ca2+ indicators Mag-fluo-4 and Fluo-4. As in toadfish, the force-pCa relation in midshipman is strongly right-shifted relative to the Ca2+ pumping-pCa relation, and contractile activity is controlled in a synchronous, not asynchronous, fashion during electrical stimulation. SR Ca2+ release per action potential is, however, approximately eightfold smaller in midshipman than in toadfish. Midshipman fibers have a larger time-averaged free [Ca2+ ] during activity than toadfish fibers, which permits faster Ca2+ pumping because the Ca2+ pumps work closer to their maximum rate. Even with midshipman's sustained release and pumping of Ca2+ , however, the Ca2+ energy cost of calling (per kilogram wet weight) is less than twofold more in midshipman than in toadfish.