5-HT4 receptors, present in piglet atria and sensitive to SDZ 205-557, are absent in papillary muscle.

Putative 5-HT4 receptors were investigated in isolated piglet left atria and papillary muscles. In atrial tissues, 5-hydroxytryptamine (5-HT) was a potent positive inotropic agent with a pD2 of 6.7. Its effects were antagonised in a selective and competitive manner by the 5-HT4 receptor antagonist, SDZ 205-557, with a pA2 of 7.3. In contrast to atrial tissues, piglet papillary muscles were virtually unresponsive to 5-HT 10(-8)-10(-3) M, suggesting that functional cardiac 5-HT4 receptors are not present in ventricular tissue.