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On their 90210MG podcast Tuesday, the actresses said they were "so shocked" to hear the news that Dowse had died at age 64 following a battle with "a virulent form of meningitis."
Prior to her death, Dowse portrayed vice principal Mrs. Yvonne Teasley on 90210, alongside Garth and Spelling, who played main characters Kelly Taylor and Donna Martin, respectively.
"This came out of nowhere. It's unbelievable to me and I can't imagine how devastated her family is, her sister, everyone who knew her. It's just shocking news," Garth, 50, said. "These kind of deaths are the hardest, I think."
Spelling, 49, added that Dowse "was so loved [by] everyone, everywhere."
Garth echoed this sentiment, noting how Dowse had "so many fans from so many different shows."
"She was in her prime, and it's so hard," Spelling said.
They went on to talk about working with Dowse on 90210, as Garth recalled, "There was just something about Denise that was so easy. She was like your oldest friend … it was so natural being around her and working with her."
"She was so easy to work with and so encouraging. She always gave the best hugs on set," Spelling added.
Garth reiterated that Dowse was "such a warm, kind woman [and] a really respected and prolific actress."
Ziering, 54, played Steve Sanders on the show — a character Garth said "worked with [Dowse] probably the most because Steve was always getting into trouble at school."
"I thought what Ian wrote was really beautiful ... because he had so many sassy scenes with her because she really had to hand it to him a lot," Spelling noted.
In his post, Ziering called Dowse's death "so heartbreaking."
"Throughout all my years working on Beverly Hills 90210, my scenes with Denise will always be remembered with the utmost in respect for her talent, and fondness for the loving soul she was," he wrote. "Some of my heartiest off camera laughs were between she and I hammering out the the discipline her Mrs. Teasley would dish out to my Steve Sanders. My sincere condolences to her family, and all others who she was dear to. God bless you Denise, pay forward that Legacy Key A."
Thinking about Dowse's passing with respect to the iconic 90's teen drama, Spelling added: "You are frozen in time with her from 30 years ago."
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On Aug. 7, Dowse's sister Tracey shared an update on Instagram that the 90210 alum had been hospitalized and was in a coma due to "a virulent form of meningitis." In the days that followed, fans and friends alike expressed their support for Dowse on social media.
By Aug. 13, Tracey had announced that her sister had sadly died. In the Instagram announcement, Tracey called the 90210 alum "the most amazing sister, a consummate, illustrious actress, mentor and director. She was my very best friend and final family member."
Tracey continued, "Denise loved all of you. I know that she is watching over us with all the love she has."
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Aside from her role on Beverly Hills, 90210, Denise also played Dr. Rhonda Pine on HBO's Insecure and Judge Rebecca Damnsen on the CBS drama The Guardian from 2001-2004.
In addition to her extensive TV credits, Denise also has starred in a number of films, including the 2004 musical drama Ray, the 1998 teen dramedy Pleasantville, and 2000's Requiem for a Dream, where she played the mother of Marlon Waynes' character, Tyrone.