Contraction and depolarization induced by fetal bovine serum in airway smooth muscle

We investigated the effects of fetal bovine serum (FBS) on the resting membrane potential and muscle tone of canine airway smooth muscles using tension recording and microelectrode methods. At concentrations >0.1%, FBS induced a sustained increase in muscle tone. At concentrations >1%, FBS depolarized the resting membrane potential of dog trachea in a dose-dependent manner. When FBS was dialyzed, it failed to induce a sustained increase in muscle tone or to depolarize the membrane, indicating that FBS contained factor(s) that induced contraction of dog airway smooth muscles. When FBS was dialyzed against distilled water, the outer solution which was freeze dried and then reconstituted (> 1% in original vol) induced a sustained increase in muscle tone, indicating that FBS contained dialyzable factor(s) that increased muscle tone and depolarized the resting membrane potential of dog airway smooth muscles. Methysergide (10^-6 M) or cyproheptadine (10^-6 M), nonspecific antagonists to serotonin receptors, 5-hydroxytryptamines (5-HT₁ and 5-HT₂), markedly reduced the FBS-induced increase in the muscle tone to ~20% of the original value but did not abolish the response. On the other hand, methysergide (10^-6 M) completely suppressed the increase in the muscle tone evoked by peak fractions obtained by application of the dialysate of FBS or 5-HT to a C₁₈ reverse-phase column, indicating the peak fractions contained only 5-HT. These observations indicate that FBS contained 5-HT and an unknown factor(s) responsible for increase in the muscle tone of the airway smooth muscle induced by FBS.