Contractile and fatigue properties of the rat diaphragm musculature duringthe perinatal period

The following two hypotheses regarding diaphragm contractile properties in the perinatal rat were tested. First, there is a major transformation of co ntractile and fatigue properties during the period between the inception of inspiratory drive transmission in utero and birth. Second, the diaphragm m uscle properties develop to functionally match changes occurring in phrenic motoneuron electrophysiological properties. Muscle force recordings and in tracellular recordings of end-plate potentials were measured by using phren ic nerve-diaphragm muscle in vitro preparations isolated from rats on embry onic day 18 and postnatal days 0-1. The following age-dependent changes occ urred: 1) twitch contraction and half relaxation times decreased approximat ely two- and threefold, respectively; 2) the tetanic force levels increased approximately fivefold; 3) the ratio of peak twitch force to maximum tetan ic force decreased 2.3-fold; 4) the range of forces generated by the diaphr agm in response to graded nerve stimulation increased approximately twofold ; 5) the force-frequency curve was shifted to the right; and 6) the propens ity for neuromuscular transmission failure decreased. In conclusion, the di aphragm contractile and phrenic motoneuron repetitive firing properties dev elop in concert so that the full range of potential diaphragm force recruit ment can be utilized and problems associated with diaphragm fatigue are min imized.