A St. Louis company that supplies an antidote to sarin nerve gas for the military and civilian first responders has suspended production due to quality-control issues. With federal regulators spending several months to evaluate and test modifications to Meridian Medical Technologies Inc.’s manufacturing process, the nation’s stockpile of the nerve-gas antidote has shrunk. Meridian has suffered a substantial loss of business. And its parent company, Pfizer Inc., is attempting to fix these problems. With production temporarily suspended, supplies of the antidote have slowly dwindled. A member of Congress has voiced concerns that the company — whose manufacturing and packaging plants in two St. Louis suburbs employ about 300 workers — was slow to inform its civilian customers about its defective products.