Studies on responses to potassium, noradrenaline, serotonin, histamine and prostaglandin F2α, of isolated pudendal arteries from non-lactating goats

Serotonin as a homeostatic regulator of lactation

Serotonin (5-HT), a neurotransmitter produced in mammary epithelial cells (MECs), acts via autocrine-paracrine mechanisms on MECs to regulate milk secretion in a variety of species. Recent studies in dairy cows reported that 5-HT ligands affect milk yield and composition. We determined the mRNA expression of bovine 5-HT receptor (5-HTR) subtypes in bovine mammary tissue (BMT) and cultured bovine MECs. We then used pharmacologic agents to evaluate functional activities of 5-HTR subtypes. The mRNAs for five receptor isoforms (5-HTR1B, 5-HTR2A, 5-HTR2B, 5-HTR4, and 5-HTR7) were identified by conventional reverse transcription PCR, real-time PCR, and in situ hybridization in BMT. In addition to luminal MEC expression, 5-HTR4 was expressed in myoepithelium, and 5-HTR1B, HTR2A, and HTR2B were expressed in small mammary blood vessels. Studies to date report that there are multiple 5-HTR isoforms in mammary tissue of rodents, humans, and cattle. Inhibition of the 5-HT reuptake transporter with selective 5-HT reuptake inhibitors (SSRIs) disrupted tight junctions and decreased milk protein mRNA expression in mouse, human, and bovine mammary cells. Selective 5-HT reuptake inhibitors act to increase the cellular exposure to 5-HT by preventing reuptake of 5-HT by the cell and eventual degradation. Increasing 5-HT concentration in milk via inhibiting its reuptake (SSRI), or by increasing the precursor for 5-HT synthesis 5-hydroxytryptophan, accelerated decline in milk synthesis at dry-off. We conclude that the 5-HT system in mammary tissue acts as a homeostatic regulator of lactation.

Adrenergic receptors in the bovine mammary artery

The response of isolated preparations of bovine mammary artery was investigated, with the aim of characterising further the adrenergic receptor subtypes present. Noradrenaline (NA) and the alpha(1) agonist phenylephrine gave sigmoidal dose-response curves with pEC(50) values of 5.97+/-0.07 (N=34) and 6.21+/-0.32 (N=8), respectively. Stimulation of alpha(2) receptors with UK 14,304 produced a weak response with pEC(50) of 6.78+/-0.38 (N=7), and maximal contraction of 17.8+/-9.9% relative to NA. A61603, an alpha(1A) agonist, gave a curve parallel to NA, but shifted to the left (pEC(50) of 6.98+/-0.19 (N=5)); this drug had an increased potency of 10-fold relative to NA, and 4-fold relative to phenylephrine. Schild analysis of curves obtained with the alpha(1) antagonist prazosin gave a pA(2) of 8.70+/-0.47 (N=6-9), whereas the alpha(2) antagonist yohimbine resulted in a pA(2) of 7.65+/-0.16 (N=4). The alpha(1A) receptor antagonists WB4101 and 5-methylurapidil gave pA(2) values of 9.39+/-0.69 (N=4) and 7.72+/-0.02 (N=2-3), respectively. The irreversible alpha(1B) inhibitor CEC reduced the pEC(50) from 5.39+/-0.12 to 4.31+/-0.18 (N=7) only at the highest dose used, and high doses of the alpha(1D) antagonist BMY 7378 produced a shift to the right at giving a pA(2) of 7.37+/-0.08 (N=3). These results suggest major involvement of the alpha(1B) adrenergic receptor subtype in contraction of the bovine mammary artery, which is similar to the human internal mammary artery.

Responses to potential vasoactive substances of isolated mammary blood vessels from lactating sows

The purpose of the investigation was to examine the response of the milk vein and the mammary artery to potential vasoactive substances in lactating sows. The response on artery and vein segments from the same sow was measured for the following substances: noradrenaline (NA), serotonin (5-HT), prostaglandin F2alpha (PGF2alpha), prostacyclin (PGI2), histamine (Hi) and potassium (K+). Interestingly, the contractile force in the mammary vein segments expressed per weight unit (mN/mg) was 1.5-2.5 fold larger than in the artery segments when NA, 5-HT and PGF2alpha were used, but similar for K+, Hi and PGI2. In vein segments, the order of sensitivity to the substances expressed by their pD2 values was 5-HT > NA = PGF2alpha > Hi > K+. The present findings suggest that NA, 5-HT, PGF2alpha, PGI2, and Hi are involved in mammary blood flow regulation in the sow, and the mammary venous system may be as important as the mammary arterial system in this regulation.