Airway smooth muscle (ASM) cell hyperplasia is a common finding in lung diseases in which increased transpulmonary pressure is necessary for adequate ventilation. The extent to which mechanical deformation of ASM cells contributes to the observed hyperplastic changes has not been addressed. To determine if cyclic stretch alters ASM cell proliferation and protein content in vitro, canine ASM cells were cultured on collagen-coated silastic membranes, subjected to a stretch-relaxation regimen, and compared with cultured cells on a rocking platform or with control (unmanipulated) cells. During the log phase of cell growth, cyclic stretch resulted in increased cell numbers (P < 0.001). Incorporation of [3H]thymidine ([3H]TdR) was assessed after 5 or 14 days in culture during 12-h pulse in stretched, rocked, and control cells. Incorporation of [3H]TdR per cell was elevated in stretched cells when compared with either rocked or control cells at 5 days but not 14 days. Total cellular protein content per well determined after cells had reached confluence was greater in stretched cells than in the controls (74.2 +/- 7.9 stretched versus 49.1 +/- 5.6 control versus 51.1 +/- 6.5 rocker; microgram/10(5) cells; P < 0.05). Transmission electron microscopic observations of stretched cells suggested increased concentrations of myofilaments compared with control cells. These observations suggest that stretch may contribute to increased ASM hyperplasia and airway resistance.