Assay—
Test animals—
Select male guinea pigs, each weighing at least 500 g, but restrict selection so that no guinea pig is more than 30% heavier than the lightest. Withdraw food, but not water, from each animal.
Sodium chloride solution—
Use Sodium Chloride Injection containing 0.9% of NaCl.
Standard preparations—
Dissolve an accurately weighed quantity of
USP Sincalide RS in
Sodium chloride solution to obtain a stock solution having a known concentration of about 10 µg of sincalide per mL. Dilute an accurately measured volume of this solution quantitatively with
Sodium chloride solution to obtain a solution containing 0.0624 µg of sincalide per kg of the animal's body weight in each 0.1 mL. Prepare a series of 1 in 2 dilutions of this solution with
Sodium chloride solution to contain 0.0312, 0.0156, and 0.0078 µg of sincalide per kg of body weight.
[NOTE—Other dose levels may be used if so indicated by the responses obtained in the
Procedure.]
Assay preparations—
Constitute 1 vial of Sincalide for Injection in a sufficient volume of Water for Injection, accurately measured, to obtain a solution having a concentration of about 1 µg of sincalide per mL. Proceed as directed for Standard preparations, beginning with “Dilute an accurately measured volume of this solution.”
Procedure—
Anesthetize each guinea pig by injecting it, subcutaneously, with 2.25 g of urethane per kg of body weight, administered as a 25% solution. Perform a tracheotomy, then expose a jugular vein, and cannulate with a polyethylene catheter. Tie a thin silk line to the free pole or fundus of the gallbladder, or attach a thin hook with connecting silk line to the wall of the fundus. Gallbladder contractile responses, transmitted through the silk line, cause a change in the line tension. Connect the free end of the silk line to a force transducer, and impose on the system an initial tension of about 2 g. Connect the force transducer to a polygraph, which records the contractile responses. Determine the sensitivity or the responsiveness of the guinea pig's gallbladder by making a few trial injections through the jugular vein catheter, then select two nonconsecutive dose levels (e.g., 0.0624 and 0.0156) for the assay. Use the same dose levels for the Assay preparations as for the Standard preparations. Administer the selected dose levels of the Standard preparations and the Assay preparations as 0.1-mL dose volumes in random order, taking 2 to 3 seconds to inject each dose volume and flushing each through the catheter with about 0.5 mL of Sodium chloride solution. Make injections at about 10-minute intervals or when the gallbladder has returned to approximately the initial 2 g of tension. [NOTE—Three injections of each dose level may be made. As many as 3 different samples can be tested on the same animal prior to retiring the animal.]
Calculation—
Calculate the potency of each vial as directed under
Potencies Interpolated from a Standard Curve (see
Design and Analysis of Biological Assays 
111
), using a log transformation, straight-line method with a least-squares fitting procedure, and a test for linearity.
