Nelson FE, Hollingworth S, Marx JO, Baylor SM, Rome LC. Small Ca
2+ releases enable hour-long high-frequency contractions in midshipman swimbladder muscle.
J Gen Physiol 2017;
150:127-143. [PMID:
29259040 PMCID:
PMC5749108 DOI:
10.1085/jgp.201711760]
[Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2017] [Revised: 07/24/2017] [Accepted: 11/22/2017] [Indexed: 01/18/2023] Open
Abstract
The swimbladder muscle of the Pacific midshipman fish contracts up to 360,000 times in an hour while generating mating calls. Using experimental measurements and computational modeling, Nelson et al. reveal the Ca2+ handling that permits these superfast muscle fibers to sustain high-frequency calling.
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.
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